Secession of the Gallic Empire

In 260, with Valerian a prisoner of Shapur I and Gallienus dealing with revolts on the Danube, Postumus — probably governor of Lower Germany — beat off a Germanic invasion. When the praetorian prefect Silvanus, who was at Cologne with Gallienus’s young son Saloninus, demanded the booty, Postumus’s troops proclaimed him emperor; they besieged Cologne, and Saloninus and Silvanus were put to death. The new state had its own consuls, praetorian guard and probably its own senate, and Postumus presented himself as the defender of Gaul and the upholder of the Roman name. He made no attempt on Italy, and an expedition by Gallienus in the mid-260s failed. By 268 the empire had split into three competing states: the Gallic Empire in the west, the Palmyrene Empire in the east and, between them, the Roman Empire proper centred on Italy.

In 269 Postumus crushed the usurper Laelianus at Mogontiacum (Mainz) but was killed by his own soldiers — reportedly because he forbade them to sack the city. Spain returned to the allegiance of Claudius II, and a force under his prefect Placidianus established itself in southern Gaul. Postumus was followed by Marius, Victorinus and Tetricus I, who in 271 moved the capital to Augusta Treverorum (Trier). In 274 Aurelian, having recovered the East from Palmyra, marched on Gaul; at the battle of Châlons the Gallic army was defeated and Tetricus surrendered. He was spared and given an administrative post in Italy.