Zosimus, New History 2.45

Zosimus (Greek Ζώσιμος): Early Byzantine, pagan author of a history of the Roman Empire, published in the first quarter of the sixth century CE.

The translation of ZosimusNew History offered here was printed in 1814 by W. Green and T. Chaplin in London, and was probably prepared by J. Davis of the Military Chronicle and Military Classics Office. The translator is anonymous. The text was found at Tertullian.org. The notes were added by Jona Lendering.

[2.45.1] Constantius, having so well succeeded in his design against Vetranio, marched against Magnentius, having first conferred the title of caesar on Gallus, the son of his uncle, and brother to Julian who was afterwards emperor, and given him in marriage his sister Constantia, either in order that he might oppose the Persians or (as seems more probable) that he might have an opportunity of taking him off. He and his brothers were the only remaining persons of the family whom Constantius had not put to death, as I have related

[2.45.2] When he had clothed Gallus with the Caesarean robe, and appointed Lucilianus general in the Persian war, he marched towards Magnentius with his own troops and those of Vetranio in one body. Magnentius, on the other hand, resolved to meet him with a larger force. He declared his kinsman caesar, and appointed him to govern the nations beyond the Alps. 

[2.45.3] The armies meeting in Pannonia, and coming near to each other at a town called Mursa, Magnentius placed an ambuscade in the defiles near to Adrana, and sent a messenger to the officers of the army of Constantius to retard their march, saying, that they might proceed to Siscia, where he intended to give them battle, the fields in that neighborhood being spacious and open. 

[2.45.4] When Constantius heard this, he was much pleased that he was to fight in a place where there was room for the cavalry to manoeuvre, being superior to the enemy in that kind of force. He accordingly led his army to Siscia. As they were marching unarmed and without order, not suspecting any thing, the troops that lay in ambush attacked them, and blocked up their passage with stones, which they threw upon them in such quantities that great part of them were killed.