Battle of Mardia
316 CE.
Shown on the map at 41.933°, 25.900°.
The Battle of Mardia, also known as Battle of Campus Mardiensis or Battle of Campus Ardiensis, was fought in late 316/early 317 between the forces of Roman Emperors Constantine I and Licinius. Its location was probably modern Harmanli (Bulgaria) in Thrace.
Text from the Wikipedia article Battle of Mardia, licensed CC BY-SA 4.0.