(Sept. 28, AD 351), defeat of the usurper Magnentius by the Roman emperor Constantius II. The battle entailed losses on both sides that severely crippled the military strength of the Roman Empire; it is known as the bloodiest battle of the century. It was also the first defeat of Roman legionaries by heavy cavalry.

In 350, having deposed the emperor Constans in the western provinces and the self-proclaimed emperor Nepotanius in Rome, Magnentius failed to gain Constantius' recognition in the eastern provinces. Constantius organized Germans on the Rhine frontier but was defeated in 351 at Atrans (modern Trojane, Slovenia). Magnentius, refusing Constantius' offer of compromise and suffering setbacks at Siscia (modern Sisak, Croatia) and Sirmium (modern Sremska Mitrovica, Yugos.), pursued Constantius to Mursa (modern Osijek, Croatia). Constantius' army was outnumbered, but after prolonged fighting his cavalry routed Magnentius' right wing, and soon his victory was complete. Losses suffered by the victors (30,000) exceeded those of the routed force (24,000), however.