Theodosius I
Civil war: 387–388

The peace with Magnus Maximus was broken in 387, and Valentinian escaped to the east with Justina, reaching Thessalonica (Thessaloniki) in summer or autumn 387 and appealing to Theodosius for aid; Valentinian II's sister Galla was then married to the eastern emperor at Thessalonica in late autumn. Theodosius may still have been in Thessalonica when he celebrated his decennalia on 19 January 388. Theodosius was consul for the second time in 388. Galla and Theodosius's first child, a son named Gratian, was born in 388 or 389. In summer 388, Theodosius recovered Italy from Magnus Maximus for Valentinian, and in June, the meeting of Christians deemed heretics was banned by Valentinian.

The armies of Theodosius and Maximus fought at the Battle of Poetovio in 388, which saw Maximus defeated. On 28 August 388 Maximus was executed. Now the de facto ruler of the Western empire as well, Theodosius celebrated his victory in Rome on 13 June 389 and stayed in Milan until 391, installing his own loyalists in senior positions including the new magister militum of the West, the Frankish general Arbogast. According to the Consularia Constantinopolitana, Arbogast killed Flavius Victor (r. 384–388), Magnus Maximus's young son and co-emperor, in Gaul in August/September that year. Damnatio memoriae was pronounced against them, and inscriptions naming them were erased.
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