Answer
In the Battle of Callinicum, Belisarius suffered a bitter defeat against a Sassanid army led by the Shahanshah Khosrau I, resulting in the loss of Roman control over the strategically crucial city of Callinicum. Despite his tactical prowess and earlier successes, Belisarius could not overcome a series of disadvantages, including a smaller army, limited resources, and the unexpected betrayal by his Arab allies during a critical moment of the battle.