Rubicon (Rubicō) is a small river in northern Italy; it flows from the Northern Apennines into the Adriatic Sea, into which it flows south of Gatteo; in the time of the Roman Republic it was the border between Italy and Cisalpine Gaul.
Due to the growing political power of Julius Caesar, the Senate set a border on Rubicon, which he could not cross with his legions. It was supposed to protect against a possible coup d’état, but Caesar crossed the river in 49 BCE (presumably on January 10) with his troops, thus starting a civil war with Gnaeus Pompey, his political competitor.
According to Suetonius, while crossing the Rubicon Caesar was supposed to say the words alea iacta est (traditionally “the dice were thrown”, and literally “the dice was thrown”), which was supposed to mean making an irreversible decision or a step of great importance.
Another famous saying related to this historic event is “to cross the Rubicon”.