Answer:
They were loyal to their tribes despite being in the Roman military.
Step-by-step explanation:
Barbarians enlisted in the military for their bravery and strength. The barbarians were tribes who were for many decades killed and torture on the hands of the Roman army. When they join the army, despite being taking the pledge to be loyal, they turn to be traitors.
One of the reasons for the fall of the Roman Empire included military weakness that helped barbarians tribes to attack and conquer the empire. Invasion by tribes of Huns and Visigoths from northern Europe defeated the Roman soldiers.