Constantius, Julian, and the Fall of Sirmium

When Julian captured Sirmium in 361, he took captive Lucillianus, normally identified as the magister equitum per Illyricum, and two legions described only as legiones Constantiacae. I argue that these units are identifiable as the Lancearii and Mattiarii which Constantius II had sent to Illyricum under his magister equitum praesentalis Arbitio ahead of his own arrival there, that the Lucillianus who was captured with these units in 361 is identifiable as the Lucillianus who commanded them again on the Persian expedition in 363, and that he was a comes rei militaris under Arbitio when captured in 361. The magister equitum per Illyricum during this period is perhaps best identified as Iovinus, and his role explained as a senior defector whose support allowed Julian to advance as fast down the Danube as he did. Finally, the magister equitum whom Julian’s forces famously captured sleeping at Sirmium in 361 is probably identifiable as Arbitio rather than Lucillianus.