The Franks were a confederation of Germanic tribes first attested in the third century AD as populating a broad strip of land on the right bank of the Lower and Middle 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 except Septimania in the 6th century. The Salian political elite was one of the most active forces in spreading Christianity over western Europe. The Merovingian dynasty, descended from the Salians, founded one of the Germanic monarchies which replaced the Western Roman Empire from the fifth century. The Frankish state consolidated its hold over large parts of western Europe by the end of the eighth century, developing into the Carolingian Empire which dominated most of Western Europe. This empire would gradually evolve into France and the Holy Roman Empire.