(d. 411) Roman emperor proclaimed in Britain
Usurper, proclaimed emperor by Roman troops in Britain. At the beginning of the 5th cent. ad Roman Britain was not heavily defended, Stilicho having withdrawn troops in 401–2 to help defend Italy against German invaders. In ad 406 Marcus seized power in Britain, but after a few months was replaced by the equally short‐lived Gratian. The next usurper, Constantine III, was more effective, managing to take substantial territories in Gaul and Spain. Eventually he was defeated by the forces of the western emperor Honorius: in 411 he was captured and executed.