Magister militum is often translated as "Master of the militia" or even "Masters of the soldiers", "chiefs of the armies", "chiefs of staff".
The command of the Roman armies was given to the magistri militum sometimes divided into magister peditum (Infantry) and magister equitum (Cavalry). The creation of this function dates back to the time of Emperor Constantine when he reorganized the armies (312-325), thus concretizing the separation of military and civil powers. The position was strategic and a strong political sign for the countries concerned.
Subsequently with the loss of power of the emperor, the magister militum became the main representative of this power. The Eastern Emperor Theodosius II officially commissioned the magister militum Castinus to govern the Roman West in his name after the death of Honorius in 423.
Fall of the Roman Empire
The Dukes (Latin:duces) of the federated peoples, adorned with this title, began to exercise a combination of powers (executive, legislative, soon temporal and spiritual through their conversion and the progressive formation of the Christian West). In Roman Gaul, Syagrius is the last Roman to bear this title surrounded by Germanic kingdoms endorsed by foedus.
From 410, the effective power of the Emperors escaped them in the Roman West.
This administrative title of the Roman Empire was therefore diverted from its meaning and led to the feudal system with its medieval titles (dux ==> duke, rex ==> king) by an evolution of the structure of power in the first kingdoms from of the Great Invasions.
His superior was the exarch, who combined civil and military powers and enjoyed considerable autonomy.
His subordinates are the military counts and the dukes (duces) who command the various army corps stationed in the Empire.