Latin: Magister militum (Latin for "master of soldiers"; : Latin: magistri militum) was a top-level military command used in the late Roman Empire, dating from the reign of Constantine the Great. The term referred to the senior military officer (equivalent to a war theatre commander, the emperor remaining the supreme commander) of the empire. In Greek sources, the term is translated either as strategos or as stratelates (although these terms were also used non-technically to refer to commanders of different ranks).
The office of magister militum was created in the early 4th century, most likely when the Western Roman emperor Constantine the Great defeated all other contemporary Roman emperors, which gave him control over their respective armies. Because the Praetorian Guards and their leaders, the Praetorian Prefects, had supported Constantine's enemy, Maxentius, he disbanded the Guard and deprived the Prefects of their military functions, reducing them to a purely civil office. To replace them, he created two posts: a commander of the infantry, the magister peditum ("master of foot"), and a more prestigious cavalry commander, the magister equitum ("master of horse"). These offices had precedents in the immediate imperial past, both in function and idea;[1] the latter title had existed since republican times, as the second-in-command to a Roman dictator.
Under Constantine's successors, the titles were also established at a territorial level: magistri peditum and magistri equitum were appointed for every praetorian prefecture (per Gallias, per Italiam, per Illyricum, per Orientem), and, in addition, for Thrace and, sometimes, Africa. On occasion, the offices would be combined in a single person, then styled magister equitum et peditum or magister utriusque militiae ("master of both forces"). Overall, lower-level magistri were assigned according to circumstances, with varying numbers employed in a given area.[2] Some were directly in command of the local mobile field army of the comitatenses, which acted as a rapid reaction force. Other magistri remained at the immediate disposal of the emperors, and by the late fourth century or early fifth century were termed in praesenti ("in the presence" of the emperor).
Over the course of the fourth century in the Western Roman Empire, the system of two imperial magistri remained largely intact, with usually one magister having paramount authority (such as Bauto or Merobaudes, the main power behind the appointment of emperor Valentinian II.) This tendency culminated in Arbogast, who inherited the position of western magister militum and used it to functionally usurp emperor Valentinian II, either killing him or driving him to suicide before appointing his own puppet emperor, Eugenius. In the west, the position (often under the title of magister utriusque militiae or MVM) remained very powerful until the formal end of the empire, and was held by Stilicho, Aetius, Ricimer, and others.
In the east, emperor Theodosius I (379-395) expanded the system of two magistri militum to include an additional three magistri. For a long time these generals were used in an ad hoc manner, being employed wherever they were needed. Eventually in the fifth century their positions became more firmly established, and there were two senior generals, who were each appointed to the office of magister militum praesentalis.
During the reign of Emperor Justinian I, with increasing military threats and the expansion of the Eastern Empire, the posts of the eastern generals were overhauled: the magister militum per Armeniam in the Armenian and Caucasian provinces, formerly part of the jurisdiction of the magister militum per Orientem, the magister militum per Africam in the reconquered African provinces (534), with a subordinate magister peditum, and the magister militum Spaniae (c. 562).
In the course of the 6th century, internal and external crises in the provinces often necessitated the temporary union of the supreme regional civil authority with the office of the magister militum. In the establishment of the exarchates of Ravenna and Carthage in 584, this practice found its first permanent expression. Indeed, after the loss of the eastern provinces to the Muslim conquest in the 640s, the surviving field armies and their commanders formed the first themata.
Supreme military commanders sometimes also took this title in early medieval Italy, for example in the Papal States and in Venice, whose Doge claimed to be the successor to the Exarch of Ravenna.
The term is referred to by Emperor Constantine VII in his De Administrando Imperio in a digression on 6th century Italian history, where he refers to mastromilis meaning 'captain-general of the army' in the 'Roman tongue'. By the time of writing in the mid-10th century working knowledge of Latin was mostly absent in the Byzantine imperial court.
By the 12th century, the term was being used to describe a man who organized the military force of a political or feudal leader on his behalf. In the Gesta Herwardi, the hero is several times described as magister militum by the man who translated the original Old English account into Medieval Latin. It seems possible that the writer of the original version, now lost, thought of him as the hereward