Latin Rubicon-, Rubico, river of northern Italy forming part of the boundary between Cisalpine Gaul and Italy whose crossing by Julius Caesar in 49 b.c. was regarded by the Senate as an act of war
First Known Use: 1626
Definition of RUBICON
river 15 miles (24 kilometers) N cen Italy flowing E into the Adriatic
Small stream that separated Cisalpine Gaul from Italy in the era of the Roman republic. The movement of Julius Caesar's forces over the Rubicon into Italy in 49 BC violated the law that forbade a general to lead an army out of the province to which he was assigned. Caesar's act thus amounted to a declaration of war against the Roman Senate and resulted in the three-year civil war that left Caesar ruler of the Roman world. Crossing the Rubicon became a popular phrase describing a step that irrevocably commits a person to a given course of action.