Ammianus, Constantius, Julian, Arbitio, Luvcillianus, Siege, Sirmium, Defection
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.