 By 400 AD, "foederati" regiments had become permanently established units of the Roman army, paid and equipped by the Empire, led by a Roman tribune and used just as Roman units were used.In addition to the "foederati", the Empire also used groups of barbarians to fight along with the legions as "allies" without integration into the field armies.Under the command of the senior Roman general present, they were led at lower levels by their own officers.