Battle of Lake Vadimo
309 BCE.
Shown on the map at 42.484°, 12.323°.
The first Battle of Lake Vadimo was fought in 310 BC between Rome and the Etruscans, and ended up being the largest battle between these nations. The Romans were victorious, gaining land and influence in the region. The Etruscans sustained heavy losses in the battle and would never again reclaim their previous glory.
Text from the Wikipedia article Battle of Lake Vadimo (310 BC), licensed CC BY-SA 4.0.