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See Christianity in Gaul for the 4th-century ecclesiastical dioceses in Roman Gaul
The diocese was established after the reforms of Diocletian and Constantine I in c. 314. In the year 407, the Rhine frontier was breached, and much of Gaul lost to barbarian tribes temporarily. Roman control over most of Gaul and the Rhineland was restored until the death of Valentinian III in 455. The territory remaining in Roman hands after the 450s was in the south in the Auvergne and Provence until ceded in 475 and in the northwest, the so-called "Domain of Soissons". After its fall to the Franks in 486 and the end of Roman administration in northern Gaul, the diocese can be said to have de facto ended.
Bibliography
P. Heather, La caduta dell'Impero romano. Una nuova storia, 2006. (in Italian)
G. Halsall, Barbarian migrations and the Roman West, 376-568, 2007. (in Italian)
As found in the Notitia Dignitatum. Provincial administration reformed and dioceses established by Diocletian, c. 293. Permanent praetorian prefectures established after the death of Constantine I. Empire permanently partitioned after 395. Exarchates of Ravenna and Africa established after 584. After massive territorial losses in the 7th century, the remaining provinces were superseded by the theme system in c. 640–660, although in Asia Minor and parts of Greece they survived under the themes until the early 9th century.
Roman Gaul - AD 400