He travelled with Ursicinus to Italy when Ursicinus was called on by Constantius to begin an expedition against Claudius Silvanus. Silvanus had been forced by the allegedly false accusations of his enemies into proclaiming himself emperor in Gaul. Ursicinus had one of his men assassinate Silvanus, restoring Gaul to Constantius. He then stayed in Gaul to help install Julian as Caesar of Gaul, Spain and Britain. Ammianus probably met Julian for the first time while serving on Ursicinus' staff in Gaul.