Why did Theodosius I allow large numbers of Teutons to serve in the Roman army?

Theodosius I allowed large numbers of Teutons to serve in the Roman army primarily because he needed to rebuild military strength and address significant personnel shortages that could not be overcome by conscription alone.1

Following the military crisis of the late 4th century, including the defeat at Adrianople, Theodosius recognized that the invading barbarian groups could no longer be expelled by force. Consequently, he sought new strategies for coexistence and defense. By integrating Teutons into the army, he aimed to bolster Roman military capacity while simultaneously recultivating tracts of wasteland through the settlement of these groups as foederati (allies).21

Under this arrangement, these groups were granted land and allowed to retain their autonomy, fighting as distinct units under their own leaders. While this policy helped stabilize the frontiers and provided necessary manpower, it was not universally popular, as it relied heavily on troops who were often culturally and religiously distinct from the Roman population.213