Rubicon, 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 bce violated the law (the Lex Cornelia Majestatis) that forbade a general to lead an army out of the province to which he was assigned. His 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 definitely commits a person to a given course of action.
The modern Rubicone (formerly Fiumicino) River is officially identified with the Rubicon that Caesar crossed, but the Pisciatello River to the north and the Uso to the south have also been suggested.