The Franks invaded the Gallic Empire when the Romano-Gallic army marched south to fight the Romans. The Romans defeated the Romano-Gallic army and reconquered most of Gaul. The depopulation and devastation of the north was so enormous, that the Romans didn't maintain a border along the Rhine for over a decade. The anarchy was simply too great, with several areas settled by Germanic Peoples from the other side of the Rhine.
Germania Inferior became dominated by Franks who fought each other for control. There was some unity among the Franks, because they allied with emperor Carausius of Roman Britain. Salian Franks used the island of the Batavians to raid the shores of Roman Britain and Gaul together with the Saxons. The first name of a Frankish king is known from this period, Gennobaudes, king of the Salian Franks in the Rhine estuary.