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Who was Magnus Maximus, and why did he challenge Roman rule in the West?

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Theodosius I Theodosius I, portrait on a gold coin from Mediolanum, Italy 378–383.

Magnus Maximus was a Roman military commander of Spanish origin who served in Britain, where he led troops against the Picts and Scots. In 383, his soldiers proclaimed him emperor, marking the beginning of his challenge to the established Roman leadership in the West.12

Maximus challenged Roman rule primarily to seize power from the existing Western emperors, Gratian and Valentinian II. After his troops declared him emperor, he crossed into the European continent, where he successfully confronted and defeated Gratian. Although he initially negotiated a period of uneasy peace with the Eastern emperor, Theodosius I, and Valentinian II, Maximus eventually invaded Italy in 387. This act of aggression forced Valentinian II to flee and prompted Theodosius I to launch a military campaign against him. Maximus was ultimately defeated by Theodosius’s forces in 388 and was executed.12