Postumus was born in a rural part of the Roman province of Germania Inferior. Little is known about his family, but it is highly probable that they were of humble origins: Postumus was known to have grown up on a small farm. He was the second of five children. Postumus was also known to be of Batavian descent. (The Batavians were a Latinized Germanic people who lived just west of the Rhine.)
Postumus enlisted in the Roman army in 235. Early on, he proved himself a competent soldier. He was also seen as loyal to his superiors. Exactly how ambitious he was during his early military career is a matter of dispute.
Postumus steadily rose through the ranks, and was promoted to general by Valerian in 254.
Governorship of Germania Inferior
Emperor Valerian appointed Postumus governor of Germania Inferior in 258. Postumus is remembered by inhabitants of Germania Inferior to have been a reasonable administrator. During his term as governor, his forces engaged in several skirmishes with Germanic tribes living east of the Rhine.
Rebellion Against Saloninus and Silvanus
In 260, Valerian waged a campaign against Persia, which resulted in his capture and execution. Meanwhile, Gallienus was in Pannonia fighting a rebellious general. At the same time, Gallienus' son and lieutenant emperor Saloninus, and Saloninus' mentor Silvanus, were in Germania Inferior. Salonius, Silvanus, and Postumus were to defend the Rhine.
On October 25, 260, Postumus intercepted an army of Juthungi that was returning home from a battle in Mediolanum (OTL 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. Postumus was immediately recognized as Emperor in the Gallic provinces (minus Narbonensis) and the province of Raetia, and he was recognized as Emperor in Spain, Britannia, and Narbonensis within two years thereafter.
After Postumus defeated Saloninus and Silvanus, he continued to act as governor of Germania Inferior for several months before appointing Julius Saturninus governor.
Tenure as Emperor
Though Postumus began his rule in 260, he did not declare the provinces under his authority to be independent of the Roman Empire until 276, during the Gallic War for Independence. Prior to the publication of the Gallic Declaration of Independence, Postumus simply regarded himself as a Roman Emperor. Even when Postumus transformed his realm into the Gallic Empire, he continued to identify with Rome: he referred to a "Two Romes" theory to justify his secession, and the new empire's formal name was the Roman Empire of Gaul.
Overall, the reign of Postumus was a characterized by internal political stability and steady economic growth.
Administrative and Constitutional Reforms
Early on in his rule, Postumus set up a system of government similar to that of the Roman Empire as it was prior to the death of Alexander Severus. For example, he established a new Senate in Colonia Agrippina. Like all of the Roman Emperors, he used a nominally republican form of government as a medium through which to exercise absolute power. The structure of the Gallic army was virtually identical to that of the Roman army. The bureaucracies inherited from the Roman Empire were largely unchanged.
During the 270s, Postumus began implementing reforms to ensure that the Gallic Empire would remain politically stable long after his reign ended. In 271, he established a clear succession procedure, something which the Roman Empire had never had. The procedure was as follows: the Emperor could designate a person to succeed him upon his death or abdication; and if the Emperor were to die or abdicate without a designated heir, the Senate would have the exclusive power to elect a new Emperor. Then in 274, Postumus declared that the Senate alone had the power to depose the Emperor, and then only after coming to an agreement on who the new Emperor should be. Postumus intended for this to be an alternative to armed rebellions against the Emperor.
Postumus implemented more drastic reforms in 278. He divided the larger provinces into smaller provinces. He grouped all of the provinces into military districts, with each military district headed by a dux; and transferred some of the military forces within each province from the command of the governor to the command of the dux. The purpose of these reforms was to keep any one person from becoming powerful enough to organize an armed uprising. He also expanded the civil service in order to allow for more rapid communication within the Gallic Empire.
Postumus laid the groundwork for the Senate to become an important party in Gallic politics. The process started when he gave the Senate a role in imperial succession. Then in 278, he gave the Senate the power to appoint the majority of the provincial governors, and thus indirect control over some of the military. In many ways, the relationship between the Senate and the Emperor was the same as it had been for almost three centuries: for example, Postumus retained the power to unilaterally appoint and dismiss Senators, and retained veto power over any act of the Senate. The Senate would not gain further power until after the death of Postumus. Yet even in the 270s, the Senate demonstrated a willingness to put its new powers to the test and openly challenge Postumus.
The Gallic Empire inherited the denarius from the Roman Empire (although by the time Postumus declared himself Emperor, the antoninianus, which was nominally worth two denarii, had all but replaced the denarius). At the time Postumus declared himself Emperor, Roman antoninianus coins were typically around 20% silver, down from around 40% in the early 240s.
During the 260s and 270s, coinage in the Roman Empire continued to degenerate. By the time of Aurelian, the antoninianus typically consisted of no more than 5% silver. Then during the Great Roman Civil War, silver was completely eliminated from the production of new coins. This was sharply contrasted by the Gallic coinage: during the entire period between Postumus' rebellion against Saloninus and the beginning of the Gallic War for Independence, the silver ratio of the antoninius remained stable at roughly 20%; although this percentage did fall to around 15% in 276 and 277.
Postumus did not issue any formal edicts on currency until 278, when he declared that the ratio of silver to copper in an antoninius should be no less than one to four. The reason that the silver content remained stable for most of Postumus' reign was simply because of Postumus' popularity: Postumus had not found it necessary to mint large quantities of coins in order to buy the loyalty of his army, unlike so many individuals who had held or sought the Roman throne during the Third Century. Postumus did allow for the Gallic currency to be debased in 276 and 277, in an effort to maintain the loyalty of his troops, during the Gallic Empire's controversial invasion of Noricum and Pannonia.
The Gallic War for Independence
Aurelian's invasion was a failure. He underestimated the loyalty of the people to Postumus. Also, Postumus ordered a devastating counter-invasion of Italy, and Aurelian was forced to send some of his troops back in order to fight off the counterinvasion. The invasion of the Gallic Empire ended in December 275, with Aurelian being deposed and succeeded by Julius Asclepiodotus, and Asclepiodotus withdrawing from the Gallic Empire immediately. The following month, Postumus formally declared the Gallic Empire to be independent from the Roman Empire.
Even though the Gallic Empire was able to repel the Roman invasion by the end of 275, the war continued for almost two more years. Postumus had made an alliance with the Alamanni, Marcomanni, and Quadi early on in the war, in which he promised them the right to annex and colonize Pannonia and Noricum if they agreed to help him fight the Romans. These three tribes agreed to invade Pannonia and Noricum, but it soon became clear that the tribes would not be able to succeed without help from the Gallic Empire. Postumus led troops to Pannonia and Noricum to help the three tribes that he had allied with, and as of 277 the Suevi as well, conquer Pannonia and Noricum. This campaign was not popular among the Gallic people, and the Gallic Senate even considered deposing Postumus before it began.
The Gallic War for Independence had far-reaching consequences. The war secured the independence of the Gallic Empire and led to the establishment of the first Germanic states. Much of Italy was devastated in the Gallic invasion. What that invasion is most commonly remembered for is the sack of Rome. The war also resulted in a complete breakdown of central authority in the Roman Empire, which led to the Great Roman Civil War.
Postumus maintained close relations with the Alamanni, Suevi, Marcomanni, and Quadi after the Gallic War for Independence. He was particularly interested in advising the tribal leaders as they summoned their civilian population to the conquered territories and began organizing states. He also kept the majority of the Gallic troops in Pannonia and Noricum as long as possible, so that they could help the Germanic tribes fight off any effort by the Romans to retake the provinces or suppress any local uprisings. The extended Gallic presence proved very helpful during the Pannonian and Norican Wars.
Postumus had far less contact with other Germanic tribes throughout his reign. Also, he had few dealings with the tribes he would eventually ally with prior to 275.
Postumus designated Victorinus as his successor late in 278. Victorinus had proven himself to be a skilled general ever since Postumus began his rule. There were several other fine generals in the Gallic army: Postumus chose him mainly because of his role in the Gallic War for Independence.
Postumus' health began to deteriorate in early 279. In October, he recalled Victorinus to Colonia Agrippina to prepare him to take control of the empire. Victorinus arrived at the capital on January 11, 280. During the following months, Postumus heavily involved Victorinus in the administration of the empire.
On May 17, 280, Postumus tripped and fell after leaving a Senate meeting. He suffered a head injury as a result of that fall, and died on May 20, 280.