Rubicon

Rubicon, a small stream in central Italy that marked the boundary separating Gaul from Italy.

Roman magistrates were not allowed to bring their armies into peninsular Italy without the permission of the senate. In 49 BC, although the senate had ordered Julius Caesar to disband his army and come to Rome alone, he marched his troops across the Rubicon. This action openly pitted Caesar against the senate and initiated civil war. "To cross the Rubicon" therefore means to take a decisive step that can never be revoked. Both the Fiumicino and the Luso rivers have been identified as the ancient Rubicon.