In 359 Constantius sent Ursicinus back to the east to help in the defence against a Persian invasion led by king Shapur II himself. Ammianus returned with his commander to the East and again served Ursicinus as a staff officer. Ursicinus, although he was the more experienced commander, was placed under the command of Sabinianus, the "Magister Peditum" of the east. The two did not get along, resulting in a lack of cooperation between the Limitanei (border regiments) of Mesopotamia and Osrhoene under Ursicinus' command and the Comitatus (Field Army) of Sabinianus. While on a mission near Nisibis, Ammianus spotted a Persian patrol which was about to try and capture Ursicinus, he was able to warn his commander in time. In an attempt to locate the Persian Royal Army Ursicinus sent Ammianus to Jovinianus, the semi-independent governor of Corduene, and a friend of Ursicinus. Ammianus successfully located the Persian main body and reported his findings to Ursicinus.