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Arch of Constantine, panel depicting the Battle of Milvian Bridge

The comitatus of Constantine

A short series of civil wars after the end of the Tetrarchy ended with the son of a Caesar, Constantine (ruled 306 - 337 AD), becoming sole Augustus. His power-base was the highest calibre army close to Italy, that of the north-western provinces. This already recruited heavily from the barbarians across the Rhine in non-Roman Germany.

New infantry regiments were formed alongside new cavalry formations, and both were closely attached to the emperor's entourage (comitatus). They became distinct from more static troops on the frontiers, both in status and strategic mobility. These entourage armies grew in size over the fourth century AD and had to be centrally supplied from state workshops. They were supposedly loyal, directly under the emperor's eye and formed a mobile reserve which could address military crises by moving quickly from region to region.

Periodically, frontier units could be promoted to the higher status of comitatenses, and disloyal regiments could be punished with re-assignment to frontier (limes) duties with the lower tier limitanei. These trends of political centralisation but increased mobility can be traced through the reforms of Severus, Gallienus and Diocletian.