 Complete the following phrase:  On other fronts, the Byzantine forces were also winning. The Persians and their Arab allies, with a mobile force of 15,000 high-quality cavalries, invaded Byzantine lands again, now via Euphratensis, a route they had never taken before. Belisarius was taken by surprise and was unsure whether this was a feint or a real attack, so at first, he did not move. He called upon Roman-allied Arab tribes for help and received 5,000 troops. He forced the Persians to retreat with a successful strategic maneuver but he kept pursuing the fleeing Persians, reportedly because his soldiers threatened mutiny if no battle was fought. With 20,000 Byzantines and 5,000 Arabs he moved against the Persians, but he was defeated by 