The situation prevailing in the fourth century after the reforms of Diocletian is shown. He created many smaller provinces and gave each, in addition to the civil governors, a dux to command the military.

Groups of provinces were commanded by a vicarius reporting to the Praetorian prefect who reported to the Emperor. In the west the Decumate had been abandoned to the Allemanni around 260 and Dacia given up under Gothic pressure in 271.