Marcus was a high-ranking soldier in Roman Britain who was proclaimed emperor by the local army sometime in 406, possibly during the summer.
[2] Possibly one of the army commanders in Britain (Comes Britanniarum, Comes Litoris Saxonici or Dux Britanniarum),[3] he may have risen to power in response to increasing raids from abroad at a time when the Empire was withdrawing troops from its far-flung provinces, such as Britain, to protect its heartland.
Bury has suggested that the revolt of the British legions in 406 was primarily directed against Stilicho,[4] the magister militum of the Emperor Honorius, the ancient sources (Olympiodorus of Thebes, Zosimus and Orosius) generally link the revolt to the barbarian incursions into Gaul and Italy, especially the Vandal and Alanic tribes crossing the Rhine, which Prosper of Aquitaine dated to 31 December 406.
Historians such as N. H. Baynes[6] and M. Kulikowski[7] argue that the rebellion was triggered by the crossing of the Rhine, which should therefore be dated to 31 December 405.
In his pseudo-historical work, the Historia Regum Britanniae, Geoffrey of Monmouth tells of a Gracianus Municeps who took the throne of Britain from King Dionotus;[13] it is possible that he based these characters on the historical Gratian and Marcus.