The river Julius Caesar crossed in 49 BC to begin civil war was the Rubicon. At the time, it marked the boundary between Cisalpine Gaul, where Caesar commanded troops, and Italy proper, where Roman generals were forbidden to bring an army without authorization.
Caesar crossed the river with the Thirteenth Legion, likely on or around 10 January 49 BC, defying the Senate’s order that he surrender his military command. Bringing an army into Italy was an act of rebellion and made armed conflict with his rival Pompey and the Senate unavoidable.
Ancient writers associate the crossing with the phrase “the die is cast,” though accounts differ over whether Caesar said it in Latin or quoted a Greek line by the playwright Menander. The modern expression “crossing the Rubicon” means making an irreversible decision. The river’s exact ancient identity was later forgotten and officially identified with Italy’s Fiumicino River in 1933.