Why could Theodosius I not expel the Visigoths from Roman territory by force?

Following the catastrophic defeat of the Roman army at the Battle of Adrianople in 378 CE, where Emperor Valens was killed and two-thirds of the eastern field army was destroyed, the Roman Empire faced a severe military crisis. The Visigoths had become an uncontrollable force within the empire, and the state lacked the military strength to forcibly expel them.12

Rather than continuing a war that the empire was ill-equipped to win, Theodosius I chose a pragmatic diplomatic solution. In 382 CE, he settled the Visigoths in Moesia and parts of Thrace as foederati (federated allies). This arrangement granted the Goths autonomy and subsidies in exchange for their service as soldiers, effectively integrating them into the Roman military structure to supplement its depleted ranks. This treaty established a model for managing barbarian tribes that the empire would rely on repeatedly as its own military capacity continued to decline.123