Marcus Cassianius Latinius Postumus was a Roman emperor of Batavian origin. He usurped power from Gallienus in 260 and formed the so called Gallic Empire. He was recognised in Gaul, Germania, Britannia and Iberia until his murder in 268.
Little is known about the early life of Postumus, but it is believed that he was a Batavian of humble origins who rose through the ranks of the army, eventually becoming the governor of Germania Superior or Inferior. While Gallienus was dealing with problems in the east, he left his son, Saloninus, and military commanders, including Postumus, to protect the Rhine. Amid the chaos of an invasion by the Alamanni and Franks, Postumus was declared emperor. Postumus then besieged and attacked Cologne where Silvanus, praetorian prefect and former co-director of Roman policy on Gaul (along with Postumus) had sided with Saloninus. After breaching the walls of the city, Postumus had Silvanus and Saloninus killed; later he erected a triumphal arch to celebrate his victory.
Postumus was recognized as emperor in Gaul, Hispania, Germany, and Britain. He set up the capital of his empire at Cologne, complete with its own senate, consuls and praetorian guard. He represented himself as the restorer of Gaul on some of his coins, a title he earned after successfully defending Gaul against the Germans. The coins issued by Postumus were of better workmanship and higher precious metal content than coins issued by Gallienus.
In 263, Gallienus launched a campaign to defeat Postumus. After initial success against him, Gallienus was seriously wounded and needed to return home. After his failed attempt at defeating Postumus, Gallienus was occupied with crises in the rest of his empire and never challenged Postumus again.
Aureolus, a general of Gallienus who was in command of Milan, openly changed sides and allied himself with Postumus. The city of Milan would have been critical to Postumus if he planned to march on Rome. For whatever reason, Postumus failed to support Aureolus, who was besieged by Gallienus.
Postumus, one of Gallienus usurpers, was himself challenged by a usurper in 268. Laelianus, one of Postumus' top military leaders, was declared emperor in Mainz by the local garrison and surrounding troops (Legio XXII Primigenia). Although Postumus was able to quickly capture Mainz and kill Laelianus, he was unable to control his own troops and they turned on him and killed him, since they were dissatisfied with him for not allowing them to sack the city of Mainz (Aur. Vict. 33.8; Eutrop. 9.9.1).
Postumus is listed among the Thirty Tyrants in the Historia Augusta. Although his reign is often listed as beginning in 259, it is now believed that the summer or fall of 260 is the more likely date that he was hailed emperor. This topic is still debated. If the date of 260 is chosen for the start of Postumus' reign, then all subsequent dates involving the Gallic Empire are pushed forward by one year.