Magnus Maximus was a Roman commander who came to imperial power in Britain in 383. This was a usurpation because Britain was part of the territory ruled by the Roman emperor of the west, Gratian, the emperor who had failed to come to the aid of the eastern emperor Valens at the Battle of Hadrianople and then had let Valens' successor, Theodosius, fend for himself in the battle's aftermath in the Balkans.
After Maximus was proclaimed emperor in Britain, he invaded Gaul. Gratian (with no help from eastern emperor Theodosius) met the usurper at Paris where Gratian's soldiers and commander-in-chief deserted him for Maximus. When Gratian fled back towards Italy, Maximus sent his cavalry commander, Andragathius, after him. Andragiathus killed Gratian in Lyons, on August 25, 383. This made Maximus ruler of Britain, Gaul, and Hispania. Maximus was accepted by the very young Emperor Valentinian II (who had been junior emperor under Gratian), as ruler of the area of the Western Empire beyond the Alps. The other (eastern) emperors were the senior augustus Theodosius and his son Arcadius.
In 387, Maximus led his army across the Alps into Italy, Valentinian's territory. Valentinian II fled with his (regent) mother Justina to Theodosius whom they appealed to for help. In 388, Theodosius led his army against Maximus, who surrendered and was executed, most likely, on July 28, 388.
Source: R. S. O. Tomlin, ‘Magnus Maximus (d. 388)’, Oxford Dictionary of National Biography, Oxford University Press, 2004 [http://www.oxforddnb.com/view/article/48295, accessed 17 Dec 2006]