The Great Roman Civil War was the longest and most destructive civil war in the history of the Roman Empire. This war was the climax of a long period of political instability in the Roman Empire. The causes of the war were the near-simultaneous but separate challenges to the rule of Emperor Aurelian by Probus and Pinianus, the emergence of Julius Asclepiodotus as a third contender for the Roman throne after his successful mutiny against Aurelian, and a clash between forces loyal to Probus and forces loyal to Asclepiodotus that quickly got out of control.
The Long Crisis of the Roman Empire
A turning point in Roman history occurred in 235 AD. That year, Alexander Severus was murdered by his own troops. His troops then proclaimed a general named Maximinus Thrax the new Emperor. Thrax was not liked by the Senate; so when Gordian, the governor of Africa Proconsularis, rebelled, the Senate was eager to back him. The rebellion failed, and Thrax prepared to march on Rome to punish the Senate. Ultimately, Thrax's invasion of Rome failed, so he met the same fate as his predecessor: murder by dissatisfied troops under his command.
Maximinus Thrax's reign set the tone for the next several decades. There had never been any formal procedure for imperial succession; and the rise of Thrax was seen by many generals and provincial governors as indicating that rebelling against the current Emperor was a perfectly acceptable means of becoming Emperor, and the message that was widely taken from downfall of Thrax was that any failure by the current Emperor or any claimant to the throne could be used as a pretext for rebellion. The following cycle dominated the next four decades: a general would gain broad recognition as Emperor, immediately have to contend a rebellion or external invasion, and in many cases be killed by his own troops, who would then align themselves with a new contender for the throne.
There were two problems that developed as a result of the political instability. One was that as Roman generals turned on each other, they had less manpower and resources to devote to defending the borders against increasingly dangerous Germanic tribes. Germanic invaders frequently raided cities throughout the empire. Another consequence of the political instability was a debasement of the Roman currency, resulting in hyperinflation: whenever a general would declare himself Emperor, he would promptly order the minting of a large supply of coins to pay his troops with.
By 260, the local authorities were beginning to take up the primary responsibility for defending their jurisdictions against external invaders. In the Syrian provinces, a general named Lucius Septimius Odaenathus was nominally loyal to Emperor Gallienus, but was in fact highly autonomous. (He died in 267, after which his wife, Zenobia, took over his realm and openly rebelled against the central government.) In western Europe, Postumus declared himself Emperor, thereby effectively founding the Gallic Empire (though he would not officially declare his dominion independent from Rome until 276).
The Gallic War for Independence
After Postumus rebelled, Gallienus made two attempts to regain control over western Europe. Both attempts failed. Gallienus' successor, Claudius II, did not recognize Postumus, but he could not challenge Postumus as he had to fight off a massive Germanic invasion in the Balkans and then died shortly thereafter. Similarly, his brother Quintillus only reigned a few months before dying, and thus did not have a chance to confront Postumus either. After Quintillus died, Aurelian succeeded him. Aurelian reconquered Zenobia's Palmyrene Empire from 272 to 273, and then he launched an invasion of the Gallic Empire in 275.
Aurelian's invasion did not go well. Aurelian underestimated the loyalty of the locals to Postumus: many civilians had enlisted in Postumus' army to fight for him, and this dramatically slowed the campaign down. Then Postumus launched a counterinvasion into Italy, which is remembered most for a brutal attack on the city of Rome and more generally for the use of scorched earth tactics in many parts of Italy. He also persuaded the Alamanni, Marcomanni, and Quadi to invade Pannonia and Noricum. The invasions of Aurelian's territories prevented Aurelian from sending reinforcements to Postumus' realm.
The slow progress of the invasion of western Europe, the counterinvasion of Italy, and the Germanic invasions of Pannonia and Noricum caused Aurelian to fall out of favor with the people and military of the Roman Empire. By the end of 275, Aurelian had been deposed, and Probus, Asclepiodotus, and Pinianus had claimed the Roman throne. Initially, each of the three candidates sought to avoid fighting each other, as they were still at war with the Gallic Empire and the Germanic tribes that had allied with it. Then in early 276, a populist republican movement in central Italy began to pose a threat to both Probus and Asclepiodotus, and both generals simultaneously decided to conquer the areas under republican control. Probus was able to take many of the republican cities and towns quickly, because he simply sent troops to central Italy from Sicily and southern Italy; whereas Asclepiodotus had to call in troops from Mauritania, who took several weeks to arrive. Many of the troops affiliated with Asclepiodotus ended up landing in cities and towns that Probus had already taken.
Course of the war
The Battles of Lavinium
The Great Roman Civil War is generally considered to have begun on June 26, 276, the date on which the Battles of Lavinium and Ostia began. On that day, troops that Asclepiodotus ordered in from Mauritania arrived in republican Italy. Some of those units landed in areas already taken by Probus. In the towns of Lavinium (OTL Pomezia) and Ostia, however, fighting broke out between the forces loyal to Asclepiodotus and those loyal to Probus. Exactly what caused the fighting in each town was unknown. The clashes resulted in Asclepiodotus' forces taking Lavinium and Probus' forces retaining Ostia.
After the Battles of Lavinium and Ostia, the commanders who had participated sent communiqués to Asclepiodotus and Probus informing them of the situation. Probus considered the possibility that the seizure of Lavinium was the result of a misunderstanding, so he ordered the troops stationed around Lavinium to try to resolve the situation peacefully. Similarly, Asclepiodotus hoped to avoid an active confrontation with Probus, so he ordered a full withdrawal from Lavinium. Unfortunately for both candidates, it took several days for the news of the Battles of Lavinium and Ostia to reach each of them, and another few days for their orders to get back to central Italy. This meant that the commanders in and around Lavinium on both sides were on their own for nearly two weeks. Asclepiodotus' commanders assumed that since Lavinium had been taken, he would want them to hold onto it; and Probus' commanders assumed that he would want Lavinium to be retaken as soon as possible. Thus, the Second Battle of Lavinium began on July 1, before either candidate even knew about the original incidents.
The Second Battle of Lavinium lasted until July 5. This battle resulted in Asclepiodotus' forces retaining the town. Probus' forces moved in on July 1. By the end of the day, it was clear to Ascleiodotus' local commander that help would be needed, so he sent for reinforcements from Sardinia. Probus' forces steadily advanced during the next two days. Then on July 4, the reinforcements from Sardinia came, and had driven Probus' forces out of Lavinium by the end of the following day.
June 276 – January 277
On July 7, 276, forces affiliated with Asclepiodotus began to move out of Lavinium to take the surrounding areas. Then on July 8, Probus' orders regarding the situation in Lavinium reached the local commanders. The commanders quickly sent out a reply explaining that Probus' orders were not relevant to the current situation, and that he had been forced into a war with Ascleiodotus. Similarly, when Asclepiodotus' orders reached the front lines on July 11, his commanders sent a reply telling him that it was too late. Probus and Asclepiodotus received their updates on July 13 and July 18, respectively.
Probus chose to throw almost his full strength against Asclepiodotus' Italian territories in order to quickly dislodge Asclepiodotus from Italy. There were more men stationed in the parts of the Roman Empire that recognized him as Emperor than there were in the areas loyal to Asclepiodotus; and unlike Asclepiodotus, Probus was not actively fighting the Gallo-Germanic alliance. On the other hand, Asclepiodotus had the benefit of already having a large number of troops in Corsica and Sardinia ready to fight, whereas Probus would have to summon forces from Africa Proconsularis, Cyrenaica and Crete, and Egypt.
Between the middle of July and early September, Asclepiodotus made excellent use of the close proximity of his forces to Italy. His forces took two swaths in central Italy: one starting from Lavinium, going through Praeneste and Corfinum, and terminating at the Adriatic Sea; and the other going up the segment of the Tiber River controlled by Probus. Asclepiodotus' army gained access to the Tiber after winning the Second Battle of Ostia on July 21 and the Battle of Rome six days later. By the middle of August, enough progress had been made on both corridors that Asclepiodotus began sending troops to take the areas in between them. Then on August 25, troops loyal to Ascleiodotus invaded Capua from Sardinia, thereby beginning the First Battle of Capua. Capua was heavily fortified at that point, so the naval invaders were not able to take the city until after reinforcements who had been making their way southward on land arrived, and even then only with difficulty. Capua finally fell to Asclepiodotus on September 6.
In early September, troops loyal to Probus began arriving in southern Italy from Africa Proconsularis. Their presence made it impossible for Asclepiodotus' forces to advance south of the Volturnus River (OTL Volturno River): Asclepiodotus' forces were fought off in the Battle of Beneventum (OTL Benevento) and the Battle of Cusae (OTL Cusa). Then on September 14, ships carrying soldiers from Egypt and Cyrenaica landed along the Italian Adriatic coast. Some of the soldiers landed in northern Italy, ready to seize it from Asclepiodotus; others landed in central Italy, not knowing that it had been brought under Asclepiodotus' control. Fierce battles ensued along the Adriatic coast. More troops came in from Probus' African territorries over the following few days. By the end of the month, Probus' forces had taken several towns and cities on the Adriatic coast, as well as Capua and Lavinium.
Asclepiodotus' situation continued to deteriorate during October. Since June, he had failed to do anything more than slow the advance of the Gallic, Alamannian, and Marcomannian armies. In northeastern and central Italy, he was continuing to lose territory to Probus. Forces affiliated with Probus also began attacking Corsica and Sardinia, and had managed to take one key city in the former and two in the latter by the middle of the month. Also during the first half of October, several lower-level commanders in Italy defected to Probus.
Out of desperation, Asclepiodotus ordered all the troops in Noricum under his command to go to Italy. He hoped to get Probus to leave him alone, and then intended to turn his attention back to Noricum. To many of the soldiers fighting in Noricum, even a temporary departure from the province was unacceptable. Some units followed the orders, but others refused to leave and defected to Probus or Pinianus. When Asclepiodotus learned that many of the troops in Noricum had defied his orders, he realized that the situation in Italy was hopeless. He left Florentium for Corsica on November 4. Units were continuing to defect, and one general had joined Probus too. Asclepiodotus ordered some of the troops that were still on his side to leave Italy. He hoped to at least secure Corsica, Sardinia, Numidia, and the Mauritanian provinces for himself in the short run. His plan was to focus on administering those provinces until the time seemed right for challenging whoever or whatever was ruling the rest of the Roman Empire. During November, his forces managed to retake the parts of Corsica and Sardinia that Probus had controlled. However, it was too late: during November, several officials who had remained ostensibly loyal to Asclepiodotus began plotting to kill him; and on November 25, he was killed.
After the death of Asclepiodotus, Flavius Antiochianus declared himself Emperor. He was recognized as such in Corsica, Sardinia, Numidia, and the Mauritanian provinces; but many of the commanders in Italy who had recognized Asclepiodotus as Emperor before October had already defected to Probus at that point, and there were too few commanders that recognized Antiochianus to effectively resist Probus. The last three Italian cities under Antiochianus' control fell to Probus' forces during the last week of December.
January – August 277
After Probus gained control of northern Italy, there was little fighting between Probus, Pinianus, or Antiochianus. During this time, Probus and Pinianus devoted most of their attention to fighting against the Gallic and Germanic invaders in Pannonia and Noricum, and Antiochianus focused on the internal affairs of his realm.
During the spring of 277, both Probus and Pinianus were able to retake parts of Noricum from the Gallo-Germanic alliance. These gains were reversed by late April, after the Suevi entered the war. Then in June and July, Pinianus suffered major defeats in Pannonia. As the news of these defeats spread throughout the Balkans and Anatolia, Pinianus began to lose support. Finally, Lucius Flavius Aper from Pinianus and declared himself Emperor on August 4. Then on August 10, a general under Probus defected to Antiochianus.
August 277 – January 280
Pinianus continued to fight the Gallo-Germanic alliance, but he was forced to redeploy troops from Pannonia in order to confront Aper. This ultimately resulted in the alliance taking the rest of Pannonia by the end of October 277. Probus had not been actively fighting the alliance since June 277.
After October 277, Pinianus turned his full attention to defeating his challenger. Aper had immediately been recognized in Moesia Inferior, Thrace, and Bithnia and Pontus, and also by several generals stationed elsewhere. In early September, he tried to take Thessalonica, which Pinianus had made his provisional capital. This effort ultimately failed: as soon as Pinianus had heard of the rebellion, he began sending large numbers of troops that were still loyal to him to Thessalonica and the surrounding area, and those forces were enough to hold off Aper's forces. For several months thereafter, neither Pinianus nor Aper made any substantial gains over the other. This changed in May 278. During the second week of the month, a weak spot was discovered in Aper's defenses in the central part of Bithnia and Pontus. Over the following week, many soldiers were sent to the area. Then on May 17, the forces loyal to Pinianus attacked Aper's forces at the weak point and broke through the line. Over the following month, Pinianus advanced westward, toward Nicomedia, the provincial capital. On June 26, Pinianus' forces took Nicomedia, arrested the governor, and installed a new governor. Thereafter, Aper's supporters began realigning with Pinianus. Aper himself disappeared on July 8 in order to avoid arrest. He was never found, and his senior staff surrendered to Pinianus on July 18.
While Pinianus was fighting Aper in Anatolia and the Balkans, Probus and Antiochianus were fighting over Italy. The governor of Africa Proconsularis and several generals stationed in Italy defected to Antiochianus in August and September of 277. Probus sent troops to retake the areas where the defecting generals in Italy were stationed. To prevent further defections, he fired or demoted several high-ranking generals who he suspected of having a potential to rebel in the future as soon as he could find suitable replacements. He also invaded Carthage (the capital of Africa Proconsularis) in November 277 to oust the governor. By January 278, he had retaken Africa Proconsularis, and he installed Lucius Valerius Messalla as the new governor. Probus and Antiochianus fought over northern and central Italy for over a year. By November 278, he had gained control over northern Italy as a result of a combination military victories by his own forces, defections by soldiers and officers in Probus' army, and desertions by some lower-ranking soldiers in Probus' army. Probus managed to hold on to southern Italy and Sicily, and central Italy remained disputed.
After Pinianus defeated Aper, he launched a campaign to retake Pannonia. He hoped that by bringing Pannonia (if not Noricum as well) back under Roman control, he would be able to keep the support he already had and win favor from generals and officials who were backing the other two imperial candidates. Similarly, Antiochianus invaded Noricum in order to prove his worthiness of the purple shortly after he had gained control of northern Italy. Thus, the Pannonian War and the Norican War began in December 278 and March 279, respectively. Pinianus and failed to make any substantial gains; and in July 278, he had to withdraw from Pannonia to face new challenges from within the Roman Empire. Once Pinianus withdrew from Noricum, some of the Gallic and Germanic soldiers who had fought Pinianus' army went to Noricum, where they rolled back the modest gains Antiochianus had made.
Pinianus' failure to regain Pannonia was the beginning of the end of his rule. In July, 279, a general stationed in the Peloponnese named Publius Aelius Aelianus declared himself Emperor and rapidly gained support throughout the Balkans. It did not take Pinianus long to realize that he would not be able to defeat Aelianus, so he surrendered to Aelianus on November 10, 279 and retired.
After Antiochianus' forces were driven out of Noricum, some of the generals and lower-ranking commanders who had supported him defected to Probus. By the end of 279, parts of northern Italy were once again disputed between Probus and Antiochianus.
Two important developments took place toward the end of 279. One was that Probus gained a foothold in the Balkans: the governor of Epirus, after abandoning Pinianus, chose to throw his lot with Probus in October. He chose Probus over Aelianus because Aelianus, while a competent military leader, was not as distinguished as Probus had been before 275. The other important event was that on November 1, Lucius Valerius Messalla, the governor of Africa Proconsularis, declared himself Emperor. Over the following weeks, Messalla gained the support of several generals and lower-ranking commanders who were dissatisfied with Antiochianus and Probus.
The year opened with the governor of Numidia becoming the latest official to throw his support behind Messalla. (This happened on January 4.) This resulted in the first expansion of Messalla's territory since Messalla began his candidacy for the throne. It also induced Antiochianus to go to war with Messalla, because Antiochianus considered Numidia part of the core of his realm.
When Antiochianus launched his invasion (in the latter third of January), Messalla was already prepared: six weeks before, he had ordered all the officers stationed in Italy, Sicily, Corsica, and Sardinia who had defected to him to come to Africa Proconsularis with the men under their command as soon as possible. He gave this order partly because he anticipated that he would soon face an invasion from one of his rivals, but also because he hoped to avoid getting bogged down in Italy like Probus and Antiochianus. This order was well received by Messalla's forces, because many of them had been summonned to Italy and the surrounding islands from the African provinces as long as four years before; and it may have had the effect of garnering Messalla additional support. Most of the forces that had aligned with Messalla were in Africa Proconsularis and Numidia by the time Antiochianus began his attack.
Antiochianus' attempt to retake Numidia was unsuccessful. The forces that Antiochianus sent there quickly found themselves outnumbered. They retreated after two days. Two days later, another group landed east of Carthage, the capital of Africa Proconsularis. This group had also been deployed by Antiochianus: Antiochianus hoped to dislodge Messalla and take over Africa Proconsularis. Unbeknownst to the forces that were assigned to take Carthage, the group that had tried to retake Numidia had landed on the opposite side of the city after being forced out of Numidia. The Battle of Carthage began on February 4, with each group affiliated with Antiochianus attacking the city only a few hours apart. Reinforcements from the surrounding areas were quickly called in. After five days of intense fighting, Messalla's forces managed to push Antiochianus' forces out of the city, but Antiochianus' forces were able to maintain a blockade of the city's harbor; and some of them went on to invade other towns and cities on the coast.
War was also breaking out in the Balkans. Aelianus invaded Epirus in the middle of January. When Probus learned of this, he diverted a minimal number of units from his Italian territories to aid the governor. He could not afford to send his ally much help, as he was still bogged down in central Italy, and he had been steadily losing men as a result of defections (mostly to Messalla, but also some to Aelianus) and desertations.
Initially, the forces Probus sent to Epirus and the men already in the province were able to hold the line against Aelianus. Then in late February, more troops affiliated with Aelianus arrived in Epirus. Over the following month, Aelianus' forces managed to gain control of the northeastern corner of the province. This steady advance was stopped on April 7, when the forces loyal to Probus successfully ambushed those loyal to Aelianus near Hadrianopolis (OTL Dropull). Many of the soldiers in those units were killed or captured, and the rest fled. Even though Probus won at Hadrianopolis, Aelianus would continue his effort to take Epirus.
In late February, Probus and Antiochianus began planning invasions of Sardinia and Sicily, respectively. Each imperial candidate knew the other was having problems with desertions and defections to Messalla, and each one was becoming desperate for an important victory against the other.
Antiochianus invaded Sicily before Probus could invade Sardinia. Forces loyal to Antiochianus invaded the city of Panormus (OTL Palerno) on March 28. Antiochianus had sent a very large invasion force to the city, as he anticipated that Probus would fight with everything he had to hold on to the island. Antiochianus was so desperate to gain a foothold in Sicily that he had even sent most of the units stationed in western Corsica, leaving that half of the island with only minimal defenses. Antiochus' gamble paid off: on April 7, Probus' forces retreated from Panormus.
For over a week, both sides prepared for further fighting. Fighting resumed on April 18, with Antiochianus' forces attempting to take towns near Panormus. Although Antiochianus' armies were able to make some gains beyond Panormus, their progress came to a complete stop by early May. For several weeks, there was a stalemate. Then on June 19, troops loyal to Antiochianus (some redeployed from the Panormus area, others sent from Sardinia) landed in an area halfway between Panormus and Lilybaeum. This was a weak area in Sicily's defenses, as Probus had been expecting Antiochianus to invade another urban area. After Antiochianus' forces took several towns, however, Probus' forces were able to adjust their positions. Probus' forces even managed to retake a couple of towns in early July.
After the Battle of Hadrianopolis, fighting had continued between Probus' and Aelianus' forces. Several towns in northern Epirus had changed hands two or three times. Probus' forces had also tried to conquer areas outside Epirus, although they had not made any significant gains.
Now it was no secret that the Roman Empire was deeply divided, and the Germanic tribes who lived north of the Danube decided that the time was right to take advantage. The Gepids and the Goths, who had been badly beaten by Roman Emperor Claudius Gothicus only twelve years before, decided to cross the Danube and invade the areas just south of the river. Their only intent was to raid small rural towns. They were still too weak to try to conquer territory or execute a massive invasion of the sort that they had staged in the late 260s.
The first few raids were conducted in late June and early July. Gothic and Gepid invaders simply crossed the Danube, raided some nearby towns, and then returned home. The Vandals conducted an invasion of their own in July.
After the Germanic tribes had carried out several successful attacks, Aelianus sent a legion across the Danube to punish the invading tribes. The Roman forces crossed the the Danube on July 21, and they returned on August 12. They had managed to recover captives that the Germanic raiders had taken, as well as some loot. Aelianus achieved this victory without having to divert troops from the areas around Epirus.
The Rise of Messalla
During the first few months after he declared himself Emperor, Messalla had made his first priority proving himself to be an effective administrator of the areas in which he was recognized, rather than trying to take over the rest of the Roman Empire as quickly as possible. He hoped to avoid wasting manpower in avoidable conflicts with his rivals, and he hoped that his decision to make administration his first priority would cause him to be seen as a desirable alternative to the other three candidates. In accordance with this strategy, he had asked all the defectors from Italy, Corsica, Sardinia, and Sicily to come to Africa Proconsularis, rather than try to fight for control of those provinces. When Numidia sided with him, however, he immediately assumed control of the province.
Messalla hoped to peacefully persuade governors and generals aligned with the other candidates to join him. This would allow him to gain control over the empire with minimal further bloodshed. Initially, he planned to wait a full year before beginning to reach out to supporters of his rivals, but he decided to start sooner after hearing about Antiochianus' initial successes in Sicily. Messalla anticipated that eventually, Probus' regime would collapse, as he was the only candidate fighting on three fronts (Italy, Sicily, and Epirus); and that thereafter, Antiochianus would gain significant momentum, which would make it more difficult for Messalla to persuade others of the necessity of defecting to him. In May, Messalla began sending out envoys to the Mauritanian provinces, Cyrenaica and Crete, Egypt, and the Syrian provinces, to persuade the governors and local commanders to defect to him. Then in June, he started sending representatives to central Italy.
Most of the generals in central Italy on both sides were very open to the idea of joining Messalla. They did not do so immediately, however, because they were concerned that the generals on the other side would not do likewise. When Messalla learned of this issue, he ordered his representatives to act as intermediaries to facilitate communication between generals loyal to Probus and generals loyal to Antiochianus, so that generals on each side could know about the intentions on those of the other side and resolve any issues that might hinder their cooperation.
Messalla's efforts to win support by means of diplomacy began to pay off when two generals who had supported Antiochianus and one who had supported Probus defected in early August. Then on August 9, the governor of Mauritania Caesariensis declared his loyalty to Messalla. During the remainder of August, all but two of the generals stationed in central Italy chose Messalla.
The Collapse of Antiochianus' Regime
By September, most of the forces in central Italy had aligned with Messalla. Messalla decided that the time was right to go to war with one of his rivals. He chose Antiochianus. Antiochianus was already preparing to attack Messalla's realm anyway, in order to recover Mauritania Caesariensis.
Messalla knew, thanks to the generals who had abandoned Antiochianus, that western Corsica had only minimal defenses. Messalla knew that Antiochianus had moved troops from Sardinia to compensate, but he decided that it would still be best to directly attack Corsica: Corsica's defenses were still significantly depleted; and if Messalla's forces were able to take the island, Antiochianus' regime would fall apart.
The invasion of Corsica began on September 9. Central Italian forces attacked western Corsica. They were joined by legions from the African provinces. As it turned out, Antiochianus' regime was already collapsing. He had ordered an invasion of Mauritania Caesariensis and Numidia; but many of the would-be participants chose to defy their orders, as they themselves came from Africa and would thus have been attacking their own homelands. Most of these soldiers joined Messalla's forces when they landed. By September 16, riots were breaking out in Aleria (OTL Aléria), Antiochianus' capital. After the riots began, Antiochianus knew his downfall was imminent and that he would likely be killed, so he hanged himself in his palace. Thereafter, Messalla absorbed what was left of Antiochianus' regime within just over a week.
The Collapse of Probus' Regime
It did not take long for word of Antiochianus' downfall to spread through Probus' territories. Reactions in Probus' realm were mixed. Some people continued to support Probus, while others began to wonder if Messalla was worth supporting. In Syracuse, Probus met with top generals to discuss how to respond to the rise of Messalla. Messalla also spent several days after absorbing Antiochianus' territories determining what to do next. For him, the question was whether to attack the core of Probus' dominion or wait for it to implode.
What Probus chose was to heavily fortify Sicily and southern Italy but avoid trying to retake any of central Italy. He did not want to provoke Messalla. Meanwhile, Messalla had chosen not to attack southern Italy or Sicily. He did not want to risk losing a large number of men in a long conflict. Thus, there was peace in Italy for a time, even though Italy was still divided.
After defeating Antiochianus, Messalla continued his efforts to gain support through diplomacy. He was especially interested in gaining the support of the governors of Achaea, Cyrenaica and Crete, Dalmatia, and Epirus, as well as the commanders of units stationed in those provinces.
Epirus did not join Messalla, but it did abandon Probus for Aelianus. Several generals stationed there had become disillusioned with Probus; and on September 28, they ousted the governor in a coup d'état and surrendered to Aelianus. When word of this reached southern Italy, the majority of Probus' generals decided that he was no longer worth supporting. While some commanders openly abandoned Probus for either Aelianus or Messalla, several generals plotted to arrest Probus, take over his regime, and then decide which of his opponents to declare allegience to (so that the transition would be orderly).
Probus knew that he was not safe in Syracuse, so he disappeared from the city during the night of October 11. His plan was to make his way to Alexandria and rule from there for the forseeable future. What happened instead was that on October 15, he was recognized and arrested in a village south of Syracuse. The locals held him for two days, after which he was taken back to Syracuse and imprisoned. The generals who had been preparing to oust Probus in a coup simply declared themselves to be in charge of Probus' regime on October 13, thus officially ending Probus' imperial candidacy.
After Probus was deposed, the group of generals who took over declared their loyalty to Messalla. Not all of the provincial governors complied with the junta, however. Sicily, southern Italy, Cyrenaica and Crete, Egypt, Arabia Petraea, and Syria Palestina sided with Messalla; but Syria Phoenicia, Syria Coele, Osroene, and Mesopotamia chose Aelianus. At the same time, two generals in Dalmatia defected to Messalla.
Aelianus Confronts Messalla
Aelianus was not willing to accept the idea of Messalla having any foothold in the Balkans, so in December he launched campaigns against the two generals stationed in Dalmatia who had taken Messalla's side. Messalla quickly sent troops to defend the generals. One of these two generals was stationed in Tarsatica (OTL Trsat), and the other was near Jadera (OTL Zadar).
Messalla sent troops to the cities where the two generals that supported him were. He also sent troops to Epidaurum (OTL Cavtat). The purpose of this was to keep Aelianus from being able to focus on just two fronts. Messalla sent a fairly large number of men to each of the three cities. He hoped for quick victories in all three cities, and hoped to take the rest of the Dalmatian coast as soon as possible thereafter.
Messalla's forces arrived at their destinations during the third week of December. The units bound for Tarsatica made their way to the city by land, while the other two groups sailed across the Adriatic. Messalla's forces succeeded on all three fronts. In Tarsatica and Jadera, the combination of the forces of the generals stationed there and Messalla's reinforcements held on to the two cities. As for Epidaurum, Aelianus did not anticipate that Messalla would attack the city for some time, so Messalla's forces achieved a somewhat easy victory. During the last week of December, the forces in Epidarum took two towns west of the city.
In January, Messalla's first priority was taking Dalmatia. The troops of his in the western part of the province were fighting to take the area between Tarsatica and Jadera. The forces who had taken Epidarum were steadily advancing westward.
During the first week of January, Messalla sent more men across the Adriatic. They were divided into two groups. One was to attack Narona. The other was to land in Epidarum and then move northeastward, conquering a strip of land extending all the way to the Danube; thereby cutting off Aelianus' forces in Dalmatia from the rest of Aelianus' realm.
The forces bound for Narona arrived at the city on January 12. As the ships had to maneuver around several of the Dalmatian coastal islands, Aelianus' forces had time to prepare for the attack. Aelianus' forces were able to withstand the attack, and Messalla's forces retreated on January 13. The forces that were meant to take Narona then sailed to Epidarum; and after they arrived at Epidarum, they joined up with the land forces who were moving toward Narona.
Meanwhile, the forces that were instructed to move northward deviated from their orders. After they landed in Epidarum, their leader decided that they should seize territory until arriving at the Tara River, then sail up the Tara and Drinus (OTL Drina) Rivers until arriving at Domavia (OTL Gradina) in northern Dalmatia, take Domavia, and then conquer a strip of land between Domavia and Epidarum. These units began moving northward from Epidarum on January 15, and they arrived at the Tara River on January 30.
It turned out that the deviation of the northbound army from its orders allowed it to intercept reinforcements Aelianus had sent to southern Dalmatia. On February 8, as Messalla's forces were moving up the Drinus River, several soldiers noticed a legion moving westward. One hundred soldiers were sent back to Epidarum to warn the forces loyal to Messalla stationed there. The forces heading toward Domavia left the Drinus in order to attack Aelianus' forces. The battle resulted in the loss of hundreds of men by Aelianus' forces. Most of the survivors managed to escape westward, while several hundred survivors returned to where they came from. As the surviving reinforcements approached the western third of Dalmatia (during late February), they found a large number of Messalla's forces waiting to meet them. The legion loyal to Aelianus was initially driven back across the Una River, but it crossed the Una River again after being joined by three auxiliary units. For a time, neither side was able to make any gains in western Dalmatia.
The forces bound for Domavia took the city on February 28. Only days later, they were confronted by forces loyal to Aelianus. During March, the Domavian forces were constantly having to defend themselves against Aelianus' forces. They did not gain any territory outside of the city until late April. During May, Messalla's forces in Domavia made only minor territorial gains.
The governor of Cyrenaica and Crete feared that Aelianus would attack Crete due to its proximity to Anatolia and the Balkans, so in January he began invading the islands closest to Crete without even asking Messalla's approval (although he did inform Messalla of his initiative). He invaded Kassos on January 10 and Karpathos on January 14. Both islands fell with little resistance. The governor then followed these easy successes by fortifying the uninhabited island of Antikythera.
By March, it was becoming apparent to Messalla that it could take a long time for his forces to win control of Dalmatia, so he instructed the governor of Cyrenaica and Crete to open a second front by invading the province of Asia. Messalla sent a legion from Africa Proconsularis to Crete to assist the forces already stationed there.
Messalla's forces gathered at Crete and Karpathos during March and early April. The campaign began on April 12, with Messalla's forces attacking the major islands of Rhodes and Kos. Kos was taken by April 16, and Rhodes was taken by April 19. Halicarnassus (OTL Bodrum) was invaded on April 20 and taken by April 23. Messalla's forces were able to take the peninsula where Halicarnassus was located, but then were unable to expand further.
On May 5, a fairly large fleet from the Cyclades approached Crete to invade it. A naval battle occurred when ships anchored in Crete went out to confront the forces from the Cyclades. Aelianus' forces were outnumbered, but several ships managed to land in the middle of the island. The governor quickly evacuated Gortyna (OTL Gortyn), the provincial capital. During the subsequent few days, naval forces loyal to Messalla formed a tight barrier around the part of Crete where Aelianus' forces had landed, and the governor made Hierapytna (OTL Ierapetra) his provisional capital. On May 10, Aelianus' forces reached the outskirts of Gortyna. During the following few days, Aelianus' forces tried to get into the city, but they failed. On May 16, they attempted to escape, as reinforcements affiliated with Messalla were approaching from both the east and west. Aelianus' forces tried to flee westward. The majority were killed before they could escape, but a minority of the forces were able to commandeer some ships and return to the Cyclades.
On May 18, an fleet of ships sailed northward from Crete. Their mission was to take over the rest of the Dodecanese and transport additional troops to the province of Asia. Several naval battles occurred during the latter third of May, with Messalla's forces attempting to gain control of the Dodecanese and the area surrounding the Ceramicus Sinus (OTL Gulf of Gökova). May ended with Messalla's forces in control of all the major islands of the Dodecanese and the city of Cnidus (OTL Tekir), and Aelianus' forces retaining control of the rest of the bay.
During June, Messalla's forces continued their efforts to take the province of Asia, but made little progress. Forces from Samos and Halicarnassus managed to capture Miletus on June 12, and gained most of the southern coast of the Ceramicus Sinus during the middle and latter thirds of the month.
On June 16, the governor of Cyrenaica and Crete sent a message to the governor of Egypt requesting reinforcements. The governor of Egypt agreed, and reinforcements arrived at Crete and the Dodecanese between July 5 and July 9. Half of the Egyptian forces were sent to the Ceramicus Sinus area, while the other half were sent to Miletus. The effects of the presence of the Egyptian forces manifested themselves quickly. During late July, the forces in Miletus moved south toward the northern coast of the Ceramicus Sinus, while the forces on the south side of the bay staged a naval attack. Messalla's forces took control of the north side of the bay on July 24. The next day, the city of Mylasa (OTL Milas) surrendered to Messalla's forces.
Throughout the civil war, the various claimants for the Roman throne had devoted the majority of their attention to warfare, and ignored the domestic affairs of their realms. Even Pinianus and then Aelianus had to a degree neglected the internal affairs of Anatolia and the Balkans. Governors had been taking increasing responsibility for the needs of the people of their respective provinces. (Messalla began taking steps to reverse this trend in his territories after overthrowing Probus and Antiochianus, however.)
By the summer of 281, the governor of Cappadocia had lost patience with Aelianus. At the same time, he was not confident in Messalla's ability to lead, and he believed that seeking the purple for himself would do nothing to help the Roman Empire's situation. On June 14, the governor of Cappadocia issued a proclamation in which he stated that he would no longer support Aelianus, he would not support Messalla or any new claimant for the emperorship, he would not declare himself Emperor in his own right, and he would assume full responsibility for the administration of Cappadocia. The next day, he sent out orders for all Cappadocian army units fighting for either Aelianus or Messalla to return to Cappadocia as soon as possible.
It took several weeks for Aelianus to learn that Cappadocia had abandoned him. When Aelianus heard what happened, he promptly ordered his commanders to take steps to prevent Cappadocian soldiers from deserting, and ordered three legions and seven auxiliary units to invade the province. Aelianus had hoped to avoid fighting on two fronts, but he believed he had no alternative. From Aelianus' point of view, the governor of Cappadocia had started down the same path toward formal secession from the Roman Empire that Postumus had taken years before, so his actions were absolutely unacceptable. Aelianus also feared that other provinces would soon follow Cappadocia if the governor's declaration of neutrality were allowed to stand.
The invasion of Cappadocia began in the middle of August, with Aelianus' forces staging naval invasions of several coastal cities and land invasions from the west and south. The northern and southern invaders were each able to take several cities before the Cappadocian forces could respond. The legion and two auxiliary unit invading from the west were less successful. They tried to enter Cappadocia at several points, only to be blocked by Cappadocian troops each time.
By September, Messalla had only managed to gain control of a third of Dalmatia, the Dodecanese, and a small part of the province of Asia. Aelianus had largely been able to hold on to his territories. Messalla was worried that he would fall out of favor with the people of his dominion if he did not make substantial gains against Aelianus in the near future.
Early in the month, Messalla sent more troops to Dalmatia. At the same time, he sent a delegation to Thessalonica, Aelianus' capital. This delegation was to propose terms for ending the war. Messalla's proposal was that Aelianus would recognize him as the sole Roman Emperor; and in return, Messalla would grant Aelianus an important role in his administration, the office of Caesar. As Caesar, Aelianus would be a subordinate Emperor, and would retain authority over most of Anatolia and the Balkans; and he would also be Messalla's successor. Messalla also sent delegations to each provincial governor that recognized Aelianus: these delegations were to inform the governors of the proposal that had been made to Aelianus and work to persuade them to support Messalla.
Aelianus refused to consider Messalla's proposal, and he had the messengers imprisoned. He was still confident that he could outlast Messalla, and he suspected that Messalla did not intend to honor his promise. Most of the governors in the Balkans were no more interested: they had Aelianus to thank for their governorships, and three of them had worked closely with Aelianus years before he had become ruler in the east. The Anatolian governors were more open to Messalla's proposal, however: most of them had been appointed by Pinianus, and they were tired of the war and wanted to see central authority re-established.
Messalla's forces won several important victories in Dalmatia during September as a result of the arrival of the additional troops. Most importantly, they took Salona (near OTL Split), the provincial capital, on September 21. In the north, the forces who had occupied Domavia for months managed to make substantial gains. By the end of the month, Messalla was in control of all of Dalmatia's major cities except for Delminium (OTL Tomislavgrad), Dioclea (near OTL Podgorica), and Scodra (OTL Shkodër). At the same time, Messalla's forces continued to steadily gain ground in Asia: by the end of September, they were in control of Laodicea (near OTL Eskihisar) and were preparing to invade Ephesus, the capital of the province.
In early October, Aelianus ordered one legion and three auxiliary units to go to Dalmatia to retake the southern coast. The legion and one of the auxiliary units, which came from Thrace and Dacia Aureliana, respectively, complied. The other two auxiliary units chose to defy Aelianus' orders. One of those units was stationed in Epirus and the other was stationed in Moesia Superior. The governors of those two provinces ordered the commanders to disregard Aelianus' orders; because they feared that Aelianus' orders would result in a cycle in which each imperial candidate would send more men to Dalmatia to counter deployments of additional forces by the other, which would result in the state of continuous violence that had only recently ended in Italy developing in Dalmatia.
The governor of Dalmatia surrendered on October 7. Most of the units based in Dalmatia recognized the governor's decision and either declared themselves neutral or declared allegience to Messalla. The majority of the units that had been sent into Dalmatia from outside chose to continue fighting for Aelianus; but with the local forces no longer willing to work with the outsiders, the outsiders could no longer effectively resist Messalla's forces. The reinforcements Aelianus sent began arriving in late October, but by that time over two thirds of the province was under Messalla's control.
The Battle of Ephesus
On October 24, Messalla's forces in Asia began an invasion of Ephesus after over a month of preparation. Two legions were sent toward the city: one from Miletus, and the other from Aphrodisias (near OTL Geyre). Both units faced heavy resistance from Aelianus' forces. The units guarding Ephesus from the south had no significant weaknesses in their line, so the legion from Miletus simply had to wear down Aelianus' defenses by launching repeated frontal attacks. The legion from Miletus broke through Aelianus' lines on October 29, but it had suffered heavy casualties by then. The legion arrived at Ephesus on October 31. Once the legion from Miletus reached Ephesus, it had to face three auxiliary units that had been stationed inside the city. These forces were able to keep the invaders from getting past the city's outskirts until November 8, when the legion from Aphrodisias finally arrived. After the legion from Aphrodisias joined the one from Miletus, Aelianus' forces found themselves outnumbered. By November 11, Aelianus' forces had to pull out of Ephesus. They set up a line just north of Ephesus. They hoped to return to the city very soon. Two auxiliary units arrived from further north on November 15. Meanwhile, one auxiliary unit loyal to Messalla joined the two legions that were already there on November 14.
Aelianus' forces launched a counterinvasion of Ephesus on November 17. Two auxiliary units attacked from the northwest, and one attacked from directly north. Messalla's forces had prepared for attacks from both directions, as well as from the northeast. Aelianus' forces did manage to enter the city from the northwest and retake several neighborhoods. Messalla's forces held on to the center of the city, however. The next day, Messalla's forces retook the neighborhoods at the edge of the city, forcing the soldiers loyal to Aelianus there to pull back to the outskirts. This left over seven hundred of Aelianus' soldiers trapped inside Ephesus, and all of them were either killed or captured. Meanwhile, the attack from the north was unsuccessful: Aelianus' forces were quickly forced to retreat. The struggle for Ephesus finally ended on November 19, when Messalla's forces made a massive frontal attack on Aelianus' lines. This assault was successful: the units loyal to Aelianus had to fall back to areas fifteen km away from Ephesus, and they sustained heavy casualties in the process.
During the beginning of November, Messalla's forces took control of the northern part of Dalmatia. Northern Dalmatia was an entirely rural area, so Messalla's forces were able to take it with little resistance. By the second week of the month, only the easternmost fifth of Dalmatia was still under Aelianus' control. That area contained the cities of Doclea and Scodra. Most of the units that had been fighting for Aelianus in Dalmatia had gathered in that area, so it would not be easy for Messalla to take over that part of the province. Because eastern Dalmatia was so well-fortified, Messalla decided to wait until later to try to take it.
Messalla sent additional troops to the Anatolian front in early November. Two auxiliary units were deployed from Italy, and one was sent from Africa Proconsularis. All three units were ordered to sail to Crete and receive further instructions from the governor of Cyrenaica and Crete. These units would not arrive at Crete for weeks, however. (At this point, he was unaware of the developments in Ephesus.)
Things were not going well for Aelianus in Anatolia. Messalla's forces were steadily gaining ground in the province of Asia. Cappadocia was steadily losing ground, but it was continuing to resist Aelianus. The naval invaders from the north had had the most success, while the invaders from the west and south had had more difficulty in retaking territory for Aelianus.
The effects of the Battle of Ephesus took several weeks to work their way through Anatolia. Initially, it seemed that the victory of Messalla's forces would be a Pyrrhic victory. Messalla's forces had taken Ephesus and the surrounding area, but they had suffered heavy casualties in the process. Also, the governor of Asia had left Ephesus for Pergamum on October 14, ten days before the battle began. Moreover, it seemed that Messalla's forces would not be able to expand beyond Ephesus for a while. Still, the fact that Messalla's forces had managed to take Ephesus was regarded as significant, as Ephesus was the capital of the province of Asia and the largest city in Anatolia to be brought under Messalla's rule.
News of the outcome of the Battle of Ephesus spread throughout Anatolia during late November and early December. A few cities and army units defected to Messalla during the first week of December. There were only a few defections at first, because it was known that Messalla's victory at Ephesus only came with heavy casualties. The defecting cities were located mainly just outside the area that Messalla's armies had taken by force; and they included Caunus (near OTL Dalyan), Cibyra (near OTL Gölhisar), and Telmessus (OTL Fethiye).
At the same time that word of the fall of Ephesus to army units affiliated with Messalla was spreading, Messalla himself was again sending out messengers to propose terms for Aelianus' surrender. Messalla's proposal was the same as it had been in September: Aelianus would retain authority over most of Anatolia and the Balkans as a Caesar, and he would succeed Messalla as Emperor. Messalla also promised the provincial governors in Aelianus' realm that they would retain their offices for no less than two years, and would not be prosecuted for having resisted Messalla. (Messalla had already chosen to retain the governor of Dalmatia following his surrender in October.)
On December 6, the auxiliary unit that Messalla had sent from Africa Proconsularis arrived at Crete. One of the units from Italy arrived on December 11, and the other arrived on December 14. The governor of Cyrenaica and Crete sent the first two of these units to Samos, which at that point was still under Aelianus' control. (The first unit to arrive at Crete left for Samos on December 9, and the second unit was sent to Samos less than a day after arriving at Crete.) The latter unit was sent to Icaria. Messalla's western forces began arriving at Samos on December 13. When they arrived, they discovered that Aelianus' forces were already fighting Messalla's local forces. The western and local units were able to take Samos by December 15. They took Icaria two days later. The third western unit arrived at Icaria the day after that.
The governor of Cyrenaica and Crete intended for the western units to help break through the lines defending Smyrna (OTL Izmir) after taking Samos and Icaria. Two of the western units sailed north of the islands on December 18, and one stayed behind to prevent Aelianus' forces from retaking the islands. The two western units that left the islands landed west of Smyrna. They had landed on the south side of a small peninsula (the OTL Urla Peninsula); and the plan was for them to cross the peninsula, sail across a small bay, march eastward upon landing, and then attack Smyrna from directly north; while local forces would attack the city from the south.
Both groups affiliated with Messalla had agreed to attack Smyrna on the morning of December 22, and they both attacked the city that morning. The invasion ended up being a failure. Forces affiliated with Aelianus were heavily concentrated around the city on all sides. Moreover, the attack from the north began before the attack from the south; so after that attack began, Aelianus' forces knew that an attack from the south was coming, and they launched a preemptive attack on Messalla's lines. The day ended with Messalla's western units fleeing for their lives, and the forces in and around Ephesus being pushed back six km.
Meanwhile, Aelianus' forces in Cappadocia suffered a disastrous defeat. The northern forces were preparing to invade the city of Sebastea (OTL Sivas) on December 21, only to find themselves ambushed by Cappadocian forces. The governor had been growing desperate for a victory against Aelianus' forces, so he had sent a full quarter of Cappadocia's military strength to Sebastea, knowing that Aelianus' forces would eventually try to take the city. Two auxiliary units tried to take Sebastea, and both were wiped out. During the battle, the Cappadocians were also able to obtain maps and communiqués pertaining to the concentrations of the forces loyal to Aelianus in various cities in northern Cappadocia.
Little happened during the remainder of December. Several minor skirmishes took place in Dalmatia, Asia, and Cappadocia, but there were no major battles. No cities or army units voluntarily changed sides either.
By early January, Messalla's forces still had yet to expand beyond the area surrounding Laodicea. This meant that Laodicea was vulnerable to an attack by Aelianus' forces. An attack came during the second week of the month.
A whole legion was sent from Bithynia and Pontus, just to retake Laodicea. Messalla's forces found themselves severely outnumbered and were forced to pull out of the city on January 11. Fortunately for Messalla's forces, they were able to evacuate the city with minimal casualties.
The soldiers from Laodicea regrouped near Aphrodisias (OTL Geyre). The legion from Bithynia and Pontus attempted to invade Aphrodisias on January 22. This invasion failed, because Messalla's forces in Aphrodisias, combined with the forces who had evacuated Laodicea, were numerous enough to resist Aelianus' forces.
Lycia and Pamphylia
Even though Messalla's forces had been driven out of Laodicea and failed to gain territory north of Ephesus, the governor of Lycia and Pamphylia chose to abandon Aelianus for Messalla on January 28. The governor was frustrated with what he considered failure on the part of Aelianus to pay sufficient attention to the needs of Lycia and Pamphylia. He was aligned with Messalla because he recalled that Messalla had chosen to spend several months proving his worth to Africa Proconsularis and Numidia before actively working to take over the rest of the empire. The governor also believed that even if Aelianus were to sucessfully drive Messalla's forces out of Anatolia, he would still have a harder time winning the war than Messalla would; so the governor believed that by defecting to Messalla, he would be helping the war end sooner.
After the governor defected, most of the units originating from Lycia and Pamphylia did likewise. The effects of this were felt quickly in Anatolia. On February 18, forces newly aligned with Messalla took part in a successful effort to retake Laodicea. On February 27, Messalla's forces began a second invasion of Smyrna. As they had more manpower this time, they were able to take the city by March 1.
While Lycia and Pamphylia was joining Messalla's side, the border province of Osroene was taking a different path. In Osroene, there was widespread dissatisfaction not only with Aelianus, but with the Roman Empire as a whole. The people of Osroene had long resented Roman rule, and were particularly angry that the various contenders for the purple had ignored their needs for years.
Riots broke out in Edessa (OTL Şanlıurfa), the provincial capital, as well as Carrhae (OTL Harran) and Batnae (OTL Suruç) during early January. The rioters targeted government officials, all of whom were either non-locals or locals who governed at the blessing of the Thessalonica regime. At first, the local army units managed to stifle the riots, but then soldiers native to Osroene began to throw their lot with the rioters. Some of these soldiers openly joined the growing uprising, while others concealed their new allegiences, so that they could stay in their units and work to sabotage them from within.
By the middle of February, the rebels were becoming more organized and better-armed. This was a result of soldiers joining the rebellion, regular contact being established between groups of rebels in different cities, and leaders emerging. By late February, the rebels had taken over several neighborhoods in Carrhae and Edessa. The rebels' success was partially due to the activities of saboteurs within the local Roman army units.
On March 3, guards at the governor's palace who were aligned with the rebels allowed rebel forces into the palace. The rebels killed many high officials in the provincial government, including the governor. The next day, the Kingdom of Osroene was proclaimed, with a merchant named Ezad Paqor bar Ma'nu as king. Intense fighting between Roman and rebel forces continued long after the takeover of the palace.
Word of the rebellion reached the governor of Galatia by late February. He chose to send two auxiliary units under his command to Osroene without even consulting Aelianus. Aelianus did not learn of the uprising until early March. When he learned about the rebellion, he sent a legion from Moesia Inferior to the province. The Galatian and Moesian units would take three and seven weeks to arrive at Osroene, respectively. Meanwhile, one of King Ezad's first acts was to send delegates to Persia to request help.
The Collapse of Aelianus' Regime
By April, Aelianus was fighting on four fronts: Dalmatia, Asia, Cappadocia, and Osroene. In Dalmatia, Aelianus' forces were in a stalemate with Messalla's forces. In Asia and Cappadocia, Aelianus had been steadily losing ground. His available forces were being depleted. A growing number of units were defecting to Messalla. Many soldiers in units still affiliated with Aelianus were either defecting to Messalla or simply deserting.
At first, it seemed like Aelianus' regime would be able to retake Osroene easily. The Roman forces outnumbered the Osrinian forces, and the Osrinian army was still poorly organized. Persian Emperor Bahram II had decided to aid Osroene however. He believed that Osroene would be a useful ally; and he believed that the Romans, after years of devastating civil war, would have neither the strength nor the will to fight the Persians. A large Persian force arrived at the Osrinian border on April 14, and quickly overran the Roman-controlled Circesium (OTL Busayrah).
May 282 was the final month of the civil war. At the beginning of the month, two legions and fourteen auxiliary units loyal to Messalla carried out a massive invasion of Pergamum, which the pro-Aelianus governor of Asia had made his provisional capital. (Since February, Asia had two governors: the one in Pergamum, who was still loyal to Aelianus; and one that Messalla had appointed, who ruled from Ephesus.) One legion and ten auxiliary units marched northwest from the area around Magnesia and Sardis, while the other legion and four auxiliary units sailed northeast from the peninsula west of Smyrna. Both groups bypassed the city of Elaea. Aelianus' generals were not expecting Messalla's forces to go straight to Pergamum, nor were they expecting so large an invasion force. On May 6, Pergamum fell to Messalla after a chaotic battle. The pro-Aelianus governor and several other high officials attempted to get out of the city, but they were killed in the confusion. This left a power vacuum in the province of Asia that only Messalla and the governor he had appointed could fill. Within ten days of the fall of Pergamum, most of the holdout cities in the province of Asia had joined Messalla's regime.
Word of the Battle of Pergamum quickly reached the Balkans. The governor of Achaea declared allegience to Messalla on May 19, and the governor of Epirus defected to Messalla on May 23. Aelianus himself surrendered on May 30. Sporadic resistance to Messalla's regime would continue throughout the summer of 282, but the Great Roman Civil War was effectively over.
Treaty of Thessalonica
Three days after Aelianus surrendered, he was placed under house arrest inside his palace at Thessalonica. This was in accordance with instructions that Messalla had issued months earlier. After Messalla learned of Aelianus' surrender, he spent several weeks determining what to do with Aelianus. He intended to keep his promise to allow Aelianus to be his Caesar, mainly because he did not want to risk provoking the governors of the eastern provinces into rebelling against him; but he believed that he would never be able to trust Aelianus. Messalla deemed it best to do everything he could to undermine and control Aelianus in the short run, and eventually maneuver him out of power or have him assassinated.
Messalla came to a decision regarding Aelianus' role in the administration of the Roman Empire by July, but he was unable to begin implementing his decision immediately as he had other matters to attend to. Finally, Messalla met Aelianus in Thessalonica on August 6, 282. There, Messalla presented Aelianus with the terms for power-sharing in the form of a treaty. Messalla informed Aelianus that the terms were non-negotiable; and that if Aelianus refused to agree, he would certainly be executed. Aelianus signed the treaty as a simple act of survival.
The terms of the treaty, known as the Treaty of Thessalonica, were as follows:
- Messalla was to be the sole Augustus (senior Emperor) in the Roman Empire. Aelianus would be the sole Caesar (junior Emperor).
- As Caesar, Aelianus would be permitted to administer the provinces of Syria Phoenicia, Syria Coele and Mesopotamia, as well as most of the provinces in Anatolia and the Balkans. The exceptions were Dalmatia, the Aegean Islands (which were to to be constituted as the new province of Insulae), Asia, and Cyprus: these provinces, as well as all the provinces outside Anatolia and the Balkans, would be directly ruled by Messalla. Aelianus was also promised authority over Cappadocia and Osroene upon their reconquest. The areas placed under Aelianus' jurisdiction were called the Dominium Caesaris (Dominion of the Caesar).
- Authority over the military in the Dominium Caesaris was to be split between Aelianus and Messalla, with Aelianus having authority over three fifths of the military forces stationed in the area, and Messalla having authority over the other two-fifths.
- Laws made by Messalla were to apply throughout the empire, and were to supersede any laws made by Aelianus. Moreover, Messalla reserved the right to nullify any law or gubernatorial or other appointment made by Aelianus; although Messalla's veto could be overriden by the unanimous concurrence the provincial governors within Aelianus' domain.
- As Caesar, Aelianus was to succeed Messalla as Augustus upon the latter's death or abdication. However, if Aelianus was found to be responsible for Messalla's death, this guarantee would be null and void, and Aelianus would be subject to removal from the office of Caesar and execution. A ten-member regency committee was to be organized. Its purpose was to investigate the circumstances of the death of the Augustus if it deemed necessary, and to exercise all the powers and duties of the Augustus until such an investigation was complete. Five of the committee's ten members were to be appointed by Messalla, and the other five by Aelianus (although Messalla reserved the right to veto any of Aelianus' appointments).