Battle of Mursa Major
351 CE.
Shown on the map at 45.560°, 18.670°.
The Battle of Mursa was fought on 28 September 351 between the eastern Roman armies led by the Emperor Constantius II and the western forces supporting the usurper Magnentius. It took place at Mursa, near the Via Militaris in the province of Pannonia (modern Osijek, Croatia). The battle, one of the bloodiest in Roman history, was a pyrrhic victory for Constantius.
Text from the Wikipedia article Battle of Mursa Major, licensed CC BY-SA 4.0.