This is an alternate timeline in which the Gallic Empire, which in our timeline was a short-lived breakaway state from the Roman Empire, manages to hold on to its independence.
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The Crisis of the Third Century
For much of the Third Century, the Roman Empire was in a constant state of civil war.
It began in 235 AD, when Emperor Alexander Severus was murdered by his own troops, who felt he was not being aggressive enough in dealing with a Germanic invasion of Gaul. Alexander Severus' forces declared Maximinus Thrax the new Emperor. Thrax was not popular in the Roman Senate, so when Gordian, the governor of the province of Africa Proconsularis, rebelled in 238, the Senate proclaimed him and his son Co-Emperors. The rebellion failed, but the Senate appointed another pair of Co-Emperors as Thrax was preparing to march on Rome to punish the Senate. The Senate's choices were unpopular among the masses, so the masses declared Gordian's grandson Emperor. This led to anarchy in the city. Meanwhile, Thrax's invasion of the city failed, so his troops killed him out of dissatisfaction.
The downfall of Alexander Severus, the reign of Maximinus Thrax, and the rebellion of Gordian I and Gordian II were the start of a vicious cycle in which a general or provincial governor would rebel against the current Emperor, possibly gain the acceptance of the Senate, immediately have to deal with an external invasion or rebellion, and in many cases be killed by his own troops, who would then align themselves with a new contender for the throne. Generals turned their attention more toward fighting each other than protecting the Empire's external borders, which led to frequent invasions by barbarian tribes. Another problem that resulted from the constant civil war was severe inflation: each time someone would declare himself Emperor, he would promptly have a large supply of coins with his face on them minted to pay his troops. This state of affairs lasted for decades.
The Rise of Postumus
Marcus Cassianius Latinius Postumus was appointed governor of the Roman border province of Germania Inferior by Emperor Valerian during the 250s. Valerian ruled from 253 to 260 alongside his son, Gallienus. After Valerian died, Gallienus reigned alone. In 258, Gallienus appointed his son, Saloninus, a Caesar (subordinate emperor).
In 260, Valerian was waging a campaign against Persia (which resulted in his capture and execution by the Persians) and Gallienus and his forces went to Pannonia to deal with a rebellious general. Postumus, Saloninus and Saloninus' mentor, Silvanus, were tasked with defending the Rhine frontier. They were stationed at Colonia Agrippina (Cologne).
Now on October 25, 260, Postumus and his forces intercepted an army of Juthungi (a Germanic tribe) that was returning home from a battle at Mediolanum (Milan). The Juthingi had lost the battle but nevertheless managed to carry off captives and other booty. Postumus's army defeated the invaders and then divided their loot among themselves. Saloninus, at the recommendation of Silvanus, ordered Postumus to hand over the loot to him. Postumus and his troops refused, and Postumus' troops declared him Emperor on November 2. Postumus and his army then laid siege to Colonia Agrippina. On December 9, Saloninus and Silvanus surrendered and allowed Postumus and his forces into the city, and were thereafter executed. Ever since then, December 9, 260 has traditionally been considered the date that Postumus' reign officially began.
Postumus was immediately recognized as Emperor in the Gallic provinces (except for Gallia Narbonensis), Germania Inferior, Germania Superior, and the Alpine province of Rhaetia. Within a year, the Britannian and Spanish provinces also aligned themselves with Postumus; and Gallia Narbonensis followed a year later. Postumus made Colonia Agrippina the capital of his realm and set up a system of government similar to that of Rome as it was before the death of Alexander Severus. The coins minted under his authority were of an unusually good quality for coins for those days. Rather than seeking control of the rest of the Roman Empire, Postumus focused on defending and bringing stability to the provinces he ruled; and he was indisputably successful at achieving this goal.
Gallienus had to leave Postumus alone for several years, as Germanic tribes were constantly invading the areas still under his control. He challenged Postumus for the first time in 265, sending Aureolus, a general loyal to him, to invade the breakaway provinces. The campaign failed as a result of carelessness on the part of Aureolus. Two years later, Gallienus himself invaded Postumus' realm, but this invasion also failed. Gallienus did manage to retake Rhaetia in the latter half of 266.
Point of Divergence
The point of divergence between this timeline and ours occurs in 268. Aureolus, who was at that time stationed in Mediolanum (Milan), openly defected to Postumus. Gallienus promptly led an invasion of Mediolanum.
What Happened in OTL
Postumus chose not to come to Aureolus' aid. Gallienus' forces laid siege to Mediolanum. During this time, Gallienus' senior officers murdered him and proclaimed Marcus Aurelius Valerius Claudius (Claudius Gothicus) Emperor. Aureolus surrendered to Claudius II, only to be executed thereafter.
Postumus' decision not to help Aureolus marked the beginning of the end of his rule. His armies became discontented with him, which is evidenced by a sudden decline in the quality of the coins issued under his authority in 268: he had more coins produced in an effort to buy the loyalty of his troops. One of his generals, Laelianus, declared himself Emperor. Laelinus was stationed in Moguntiacum (Mainz), so Postumus led forces that were still loyal to him to invade Moguntiacum. Postumus defeated Laelinus, but then forbade his soldiers to plunder the city. For this, they murdered him and proclaimed Marcus Aurelius Marius the new Emperor.
Thereafter, the territory Postumus had controlled fell into the same state of disorder that the Roman Empire as a whole had fallen into more than thirty years before. Postumus' realm became increasingly weak; and in 274, Roman Emperor Aurelian reconquered what was left of it.
What Happened in This Timeline
Postumus decided to lead a campaign to Mediolanum to help Aureolus out. The combined forces of Postumus and Aureolus managed to repel Gallienus' campaign. Gallienus' senior commanders, who wanted him dead anyway, murdered him and proclaimed Marcus Aurelius Valerius Claudius the new Roman Emperor.
The Remainder of the Third Century
For detailed account, see Timeline: Late 3rd Century.
The Remainder of the Reign of Postumus and the Great Roman Civil War
For several years after the Battle of Mediolanum, there were no confrontations between Postumus’ dominion and the rest of the Roman Empire, although the governor of Raetia did align with Postumus in 269. After the death of Gallienus, Claudius II, the new Roman Emperor, had to immediately turn his attention to defeating a massive naval invasion of the Balkans and the Aegean Sea by various Germanic tribes. Claudius died of smallpox in January 270; and by September of that year, Aurelian had emerged as the new emperor. During the early 270s, Aurelian contended with Germanic invasions and a rebellion in the eastern provinces and oversaw several domestic reforms, but he also prepared for a massive campaign to retake the western provinces. As for Postumus, he, too prepared for war.
Aurelian began a three-front invasion of Postumus’ realm in March 275. Initially, Aurelian’s invasion was a success; but by the middle of May, the invasion had been completely crushed on one front and had stalled on the other two. Shortly after the war began, Postumus sent envoys to several Germanic tribes to ask them for their aid in exchange for the Roman provinces of Pannonia Inferior, Pannonia Superior and Noricum. The Alamannians, Marcomannians and Quadians accepted the offer, on the condition that Postumus successfully invade another part of Aurelian’s realm. In order to persuade the Germanic tribes to join the fight, Postumus sent several legions loyal to him to Italy to conduct a scorched earth campaign and sack Rome. The invasion was a success: Italy was devastated, the Germanic tribes Postumus had reached out to were convinced to join the fight, and Probus and Pinianus usurped the emperorship. Finally, at the end of 275, Postumus’ forces retook southern Spain; which led Aurelian’s remaining supporters to turn on him, depose him, and recognize Julius Asclepiodotus as the new emperor. After the downfall of Aurelian, Asclepiodotus withdrew from Postumus’ realm. Shortly after that, Postumus declared the territory under his control an independent state, known as the Gallic Empire. In spite of these developments, the Gallic War for Independence continued until October 277, because Postumus chose to assist the Alamannians, Marcomannians, Quadians, and later Suevians in conquering Pannonia and Noricum.
At the same time that the Gallo-Germanic invasion of those provinces was taking place, a civil war, which would be known as the Great Roman Civil War, erupted in what was left of the Roman Empire. The three generals who had declared themselves emperor in the wake of the downfall of Aurelian — Probus, Asclepiodotus and Pinianus — began fighting one another for control over the entire empire. By November 276, Asclepiodotus had fallen out of favor in his regime, so he was killed and replaced by Antiochianus. For four years after that, control over central and northern Italy would repeatedly switch between Probus and Antiochianus. Pinianus faced two rebellions in his realm: the first was by Aper, and the second was led by Aelianus. Pinianus was able to defeat Aper, but he could not defeat Aelianus. At the end of 279, Messalla rebelled against Probus and declared himself emperor. Within a year, Messalla managed to force Probus and Antiochianus out of the war and take over their territories. The Great Roman Civil War continued until May 30, 282, when Aelianus surrendered to Messalla. Before the war ended, two eastern provinces, Cappadocia and Osroene, broke away from the empire.
After the Gallic War for Independence ended, Postumus continued to maintain close relations with the nascent Germanic states of Alamannia, Marcomannia, Suevia, and Quadium. Also, he made several laws that changed the Gallic Empire’s administrative structure and gave more power to the Gallic Senate. Postumus died on May 20, 280, and was succeeded by a general named Victorinus.
The Reign of Victorinus
Early in Victorinus’ reign, the Gallic Senate considered deposing him after it learned that Victorinus had secretly been attempting to prolong the Gallic Empire’s military presence in Pannonia and Noricum. By a narrow margin, the Senate voted against deposing Victorinus. This scandal set the tone for Victorinus’ relationship with the Senate: many senators would be openly hostile to him, but he would casually dismiss such senators from the body and replace them with trusted allies whenever doing so was convenient for him. Thus, Victorinus’ relationship with the Senate was different from that of Postumus: Postumus believed in genuine co-operation between the Senate and emperor, while Victorinus had no qualms about reducing the Senate to a puppet.
Victorinus built on Postumus’ foreign policies. He maintained close relations with the four Germanic states in Pannonia and Noricum, and he also developed diplomatic and trade relations with other tribes. He eventually did end the Gallic military presence in Pannonia and Noricum. Victorinus also tried to forge economic and cultural ties with the Roman province of Mauritania Tingitana, as he hoped for the Gallic Empire to eventually annex the province. For most of his rule, Victorinus restrained himself from debasing the currency. He also presided over a project to codify the laws of the Gallic Empire. He injected Celtic themes into the state religion.
Victorinus died on December 6, 293. He failed to designate a successor, so the responsibility of choosing the new emperor (now known by the title of First Citizen) fell on the Senate. After months of intense debate and campaigning, the Senate elected Tetricus First Citizen by a plurality vote. Tetricus named Carausius as his successor.
The Reign of Messalla
After Messalla won the Great Roman Civil War, he made a power-sharing agreement with Aelianus. Under the terms of the agreement, Aelianus would rule over an area that included the majority of the Anatolian and Balkan peninsulas as a subordinate emperor, and would succeed Messalla as the senior emperor. Messalla initially offered Aelianus this deal in an effort to persuade him to surrender, but he also intended for the agreement to permanently serve as a procedure for imperial succession. By the time the war was over, Messalla decided that he could not trust Aelianus, so he did everything he could to minimize Aelianus’ role in governing the empire, and he eventually had Aelianus killed. After the death of Aelianus, Messalla appointed Maximian the new Caesar. Messalla trusted Maximian and was willing for Maximian to eventually succeed him, so he granted him true autonomy in the provinces that he was assigned to govern.
Rule with Aelianus
Messalla’s most important domestic policy during the early years of his rule was setting up a massive public works program to rebuild cities and infrastructure that had been destroyed or damaged during the war. The program did succeed in rebuilding many cities, but the taxes and inflation needed to fund the program were so high that the economy of the empire remained stagnant for a long time.
In 283, Messalla disbanded the Roman Senate, as he considered it a potential threat to his power. After the Senate was dissolved, most of the senators were given new responsibilities and allowed to retain the title of senator. The term "senator" was redefined: instead of referring to a member of a legislative body, it now referred to a holder of any high-level office. Messalla himself took the title of Senator Supremus (Supreme Senator) in 285.
An important event in Messalla’s rule took place in 286. He and Aelianus agreed to invade the breakaway province of Osroene. They also agreed to pre-emptively attack Persia, with which Osroene had aligned. The effort to retake Osroene was a disastrous failure: Persia and Osroene fought off the Roman invaders and forced Rome to cede territory to Osreone and pay reparations to both Persia and Osroene. Also, Armenia had entered the war on Rome’s side, and it too was defeated and forced to pay reparations and cede territory, and it was also forced to adopt a neutral position in all future Perso-Roman conflicts.
The reparations the Roman Empire was forced to pay were mainly food, livestock, horses, and tools, and they came mainly from the Anatolian provinces. These provinces had escaped most of the destruction of the civil war (except for the province of Asia), but the confiscation of goods for the purpose of transferring said goods to Persia devastated their economies and led to severe food shortages. This in turn led to sustained civil disorder in the eastern provinces. These provinces were all within Aelianus’ territory, and Messalla used the unrest as a pretext to invade Aelianus’ realm and have Aelianus deposed. Messalla’s forces began to invade Aelianus territory in November 288, and they quickly overwhelmed Aelianus’ armies. On November 29, Aelianus surrendered and committed suicide.
Rule with Maximian
After the death of Aelianus, three generals provisionally took control of the area he had ruled. Maximian was appointed the new Caesar on March 22, 289. Messalla had considered appointing Diocles Valerius, a longtime friend of Maximian, but he chose not to appoint Diocles because he believed Diocles was too ambitious. Even though Diocles was not chosen to be the new Caesar, he became very influential on both Messalla and Maximian.
During the early 290s, Diocles formulated plan to overhaul the Roman Empire's administrative structure. The proposal called for the division of most provinces into smaller provinces, the creation of two levels of government between the imperial and provincial level, and the division of control over the military between the provincial governors and the heads of the new subdivisions. Messalla began implementing this plan in 292. Diocles also proposed drastically expanding the imperial bureaucracy. This proposal was adopted by Maximian, but not by Messalla.
Another important reform during the 290s was the replacement of the denarius with five new coins.
During the late 290s, Messalla and Maximian began preparing to reconquer Cappadocia. In order to provide an incentive for men to enlist in the army, Messalla minted gold coins only to pay soldiers, and began minting a new type of silver coin worth two regular silver coins. Many young men began enlisting after this decree was made.
Ever since the 270s, Christianity had been growing rapidly in the Roman Empire. By the 290s, the imperial government was beginning to take notice. Diocles began urging Messalla and Maximian to combat the spread of Christianity. Messalla barred Christians from serving as governors, praetorian prefects, or Caesar. Maximian and some governors and praetorian prefects carried out broader purges of the army and bureaucracy. More severe state actions against Christians would follow.
The Reign of Tetricus I
Tetricus I was able to win the election to the emperorship because he promised to treat the Senate as an equal partner in governing the Gallic Empire, and because he promised to immediately declare Carausius his successor. After he was elected, he honored the letter of his promise, but he immediately began marginalizing Carausius. Tetricus I kept his promise to deal with the Senate as an equal, and thus restrained himself from casually stuffing the Senate with his partisans.
The Gallic Empire fought two wars during the 290s. From May 295 to April 296, Tetricus I sent Gallic troops to drive Berber invaders out of Mauritania Tingitana. Tetricus and the Senate opted to permanently incorporate the area south of Mauritania Tingitana into the Gallic Empire as the province of Amasiga. This war was known as the Amasigan War. The other war took place north of the Danube, and it was called the First Burgundian-Aquilonian War. By the late 280s, it was clear that some of the Alamannians, Suevians, and Marcomannians intended to stay in their old homelands, and they had become collectively known as Aquilonians (Northerners). At the same time, the Burgundians and branches of other Germanic tribes settled into the now-depopulated area. Relations between the remaining old inhabitants and the newcomers were tense, and degenerated into warfare by late 297. In early 298, the Gallic Empire, Alamannia and Marcomannia sent troops to protect the Aquilonians.
During the 290s, the governors of four provinces in northern Gaul began making Gaulish equal to Latin as an administrative language. The effects of this would manifest themselves a generation later, with the beginning of the Celtic Renaissance.
The Fourth Century
For detailed account, see Timeline: 4th Century.
Roman Reconquest of Cappadocia
In May 304, Roman co-emperors Messalla and Maximian launched an invasion of the breakaway province of Cappadocia after spending several years working to rebuild the Roman army. By October, the governor surrendered and recognized Messalla and Maximian. Messalla and Maximian allowed the governor to retire peacefully.
The Assassination of Tetricus I and the First Gallic Civil War
In 304, Carausius began conspiring with associates of his to have Tetricus I assassinated so that he could become First Citizen of Gaul. The assassination took place on September 8, 304. Carausius immediately assumed the throne. Many senators suspected that Carausius was involved and began investigating the assassination of Tetricus I. They eventually uncovered evidence that Carausius was involved and persuaded the most senior senator to call a meeting to depose Carausius. The senior senator agreed to call such a meeting on February 16, 305 after sunset. At that meeting, the Senate voted to depose Carausius, name Arpagius as the new First Citizen, and designate Tetricus' son as Arpagius' successor. Carausius refused to recognize the Senate's decision on procedural grounds, and he declared the senators who had voted to depose him to be expelled from the Senate. By February 24, this led to the outbreak of the First Gallic Civil War.
During March and April of 305, military and civil officials throughout the empire sided with either Arpagius or Carausius, and both contenders for the throne began preparing to fight each other. During the summer of 305, there were two main fronts in northern Gaul. Meanwhile, the governor of Britannia Inferior and the dux of Britannia fought a parallel war with the governor of Britannia Superior for control over the island. Ultimately, Arpagius prevailed over Carausius. Carausius was disadvantaged from the start; and as the war progressed, he only lost territory. On November 8, 305, Carausius was arrested by his former supporters. The last resistance to Arpagius was defeated on January 5, 306. Carausius was executed on January 30, 306.
The Reign of Arpagius
The Third Gallo-Roman War
After Messalla learned about the civil war in Gaul, he decided to use the Gallic Empire's state of internal division as an opportunity to reconquer the Germanic states of Pannonia and Noricum. He ordered a massive invasion of the region in late September 305. Within weeks, Suevia was completely overrun, and Marcomannia and Quadium also lost much territory. The governments of all four states appealed to Arpagius for help. By the time Arpagius received the requests for military intervention, Carausius had recently been arrested, so Arpagius decided to aid the Germanic states.
Arpagius sent troops to Pannonia and Noricum, and they began arriving in January 306. These units joined the local forces in fighting the Romans. Arpagius also deployed multiple legions to stage a massive attack on the Roman Empire itself. Because of a Gallic presence in Pannonia and Noricum and the need for Roman troops in the area to be recalled to combat the invasion of the Roman Empire itself, the Germanic states were able to win back territory. The war effectively ended on June 15, 307 with an armistice agreement. Under the terms of the agreement, the Gallic Empire would be allowed to annex the Roman province of Mauritania Tingitana, while the Roman Empire would be allowed to annex the parts of Pannonia and Noricum south of the Savus (OTL Sava) River.
Even before the war was over, Arpagius ordered the generals leading the invasion of Mauritania Tingitana to begin integrating the province into the Gallic Empire. This process was completed by 308.
Relocation of the Capital
In November 308, Arpagius proposed choosing a new capital city for the Gallic Empire. After several months of deliberation, the Senate voted to designate Tricassium (OTL Troyes) as the new Gallic capital city on March 25, 309. Construction of a new imperial palace began soon afterwards, and Arpagius and the Senate moved to Tricassium in September 319.
After the Third Gallo-Roman War, Arpagius expanded the size of the Gallic army. He also implemented two major reforms to the military. His first reform was altering the structure of the army's units. The other major change was dramatically expanding the Gallic navy.
Beginning of the Celtic Renaissance
During the 290s, the governors of Aremorica, Lexovia, Aulercia, and Pictonia placed Gaulish on par with Latin as an administrative language. The governor of Belgica made Gaulish an administrative language in 302, and the governor of Senonium would do likewise in 303.
The effects of the policy on administrative languages began to manifest itself in the 320s. By that time, it had become necessary for the provincial elites to be able to understand Gaulish. Government documents were written in both Latin and Gaulish in the Gallic Empire's northern continental provinces. Also, privately-written texts that were originally written in Latin or Greek were being translated into Gaulish. Then in 324, a Belgican author wrote the first book to be originally written in Gaulish (it would not be translated into Latin for more than two decades).
The 320s marked the beginning of the Celtic Renaissance. This would be a time period during which the culture of the northern parts of the Gallic Empire would become less Latin and more Celtic. Although this cultural shift was concentrated in northern and central Gaul, it would later spread to Britannia, and have a limited influence on the empire as a whole.