The Franks were a confederation of West Germanic tribes, 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. The name France is derived from the Franks.
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 or Rome. Instead, they followed their own line of Popes which the Byzantines considered heterodox.
By 769, the King of the Franks retained independence from both the Pope and the Caliph. Caliphate diplomat Da'ud ibn Zubayr feared the presence of Christian missionaries in the Bulgar nation, as a Christian Bulgar nation would be likely to ally with the Franks and create a coalition posing a serious threat to the Caliphate.