Franks

The Franks were a West Germanic tribal confederation first attested in the third century as living north and east of the Lower Rhine River. From the third to fifth centuries some Franks raided Roman territory while other Franks joined the Roman troops in Gaul. Only the Salian Franks formed a kingdom on Roman-held soil that was acknowledged by the Romans after 357. In the climate of the collapse of imperial authority in the West, the Frankish tribes were united under the Merovingians and conquered all of Gaul save Septimania in the 6th century. The Salian political elite would be one of the most active forces in spreading Christianity over western Europe.

Franks in Agent of Byzantium
The early success of the Franks was set back in the Fifth century when the Byzantine Empire reasserted itself. The border stabilized with the Franks, along with other Germanic tribes, eventually forming the Franco-Saxon kingdoms in north-western Europe.

While Christian, the Franks did not acknowledge the authority of Constantinople. Instead, they followed their own line of Popes which the Byzantines considered heterodox.