Postumus revolted in either 260 or 259 and became the emperor of the Roman-Gallic Empire. He was proclaimed as emperor by the Roman Rhine armies, and acknowledged by Germania Inferior, Germania Superior, Britain, Gaul and Spain. He didn't marched to Rome to claim to become emperor of the Roman Empire, like previous usurpers. Instead he made Colonia Agrippina as his capital.
The Roman empire was in great turmoil due to several foreign invasions. The Franks and the Alamanni had devastated Germania Inferior, Germania Superior and northern Gaul, and the Franks even reached Spain. Postumus was a Batavian living on the other side of the Rhine from the Franks and probably wanted to protect his own region from its own enemies. He was still fighting the Frankish invaders to oust them from Roman territory, when he became emperor. The son of Roman emperor Gallienus, Saloninus, was in Colonia Agrippina and he was murdered by the Rhine armies during the revolt.
After the victory on the Franks, Postumus turned against the Alamanni, defeating the Alamannic Juthungi, whom were in control of Raetia in the early 260s.
Gallienus attacked the Gallic Empire in 263, first advancing into Raetia, which was reconquered by the Romans and than into Gaul. General Aureolus was supposed to lead the Roman attack into Germania Superior, but he ceased the assault. Gallienus managed to besiege Postumus in an unnamed Gallic city. We don't know the name of this city thanks to the severe lack in sources. At this city he got wounded and called of the attack and went back to Rome and the Gallic Empire was saved. The Alamanni got a definite hold on the Agri Decumates.
Aureolus was governor of Raetia, protecting the northern borders of the Roman Empire versus the Allamanni, when he revolted against Gallienus in 268. He marched to Mediolanum in Italy and asked Postumus for support. But, Postumus remained in Germania. The Allamanni immediately invaded Raetia. Gallienus besieged Mediolanum, but was killed by his own soldiers. Aureolus surrendered to the new Roman emperor Claudius II.
In the meanwhile, Postumus had problems for his own, because the Gallo-Roman army in Germania Superior revolted under Laelianus. Postumus conquered the capital of the province, Moguntiacum and killed Lealianus, but he was subsequently killed by his own troops as well.