Why did Theodosius I go to war against Magnus Maximus in 388?

Theodosius I initiated military action against Magnus Maximus in 388 primarily because Maximus invaded Italy in 387, forcing the young co-emperor Valentinian II to flee to Thessalonica.12

Previously, Theodosius had refrained from challenging Maximus due to a lack of sufficient military strength and the need to secure his own borders. However, by 388, Theodosius’s position had significantly improved following a successful treaty with the Persians that ensured peace on his eastern frontier. Additionally, his decision to intervene may have been influenced by his marriage to Galla, the sister of Valentinian II, at the end of 387. Following the invasion of Italy and the subsequent incursion into Pannonia, Theodosius launched a campaign that defeated Maximus’s forces by July 388, leading to the usurper's capture and execution in August.12