How did Theodosius I defeat Magnus Maximus?

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Theodosius I defeated the usurper Magnus Maximus in 388 following Maximus's invasion of Italy and the displacement of Valentinian II.1

After securing peace on his eastern border through a treaty with the Persians, Theodosius organized a multi-pronged military response. He dispatched an army division from Egypt to Africa and sent Valentinian II with a fleet to Italy. In the spring of 388, Theodosius led his main force against the army of Maximus, which had invaded Pannonia.1

By July 388, Theodosius had successfully defeated the enemy forces. Maximus surrendered at the end of August and was subsequently branded a usurper, though his followers were largely treated with leniency. Following the victory, Theodosius restored the West to Valentinian II while ensuring his own influence by appointing the general Arbogast as a trusted adviser to the young emperor.1