BATTLE OF THE MILVIAN BRIDGE
312 AD

At the Battle of the Milvian Bridge, also called Mulvian Bridge, which by the way crosses the river Tiber, Constantine defeated Maxentius who drowned while trying to escape.

With Maxentius out of the way, Constantine was now the uncontested ruler in the West.

Legend has it that it was before this battle that Constantine saw the cross in the sky and the instruction In this sign conquer. Thus he painted the logo on all the shields of his men. Something like this:

And here is the map

LOCATION OF THE MILVIAN BRIDGE
Click to enlarge

This map also shows the location of the Milvian Bridge.