Sirmium [Mitrowitz on the Save] was a city of lower Pannonia, not far from the Danube, and was the great bulwark of the Illyrian provinces of the Empire. There Vetranio assumed the purple; and there Constantius was born. The frontier war caused it to be from time to time the Imperial residence. We hear of Constantius at Sirmium in the summer of 357. Ammian. xvi. 10. He also passed there the ensuing winter. ibid. xvii. 12. In October, 358, after the Sarmatian war, he entered Sirmium in triumph, and passed the winter there. xvii. 13 fin. and with a short absence in the spring, remained there till the end of May, 359.