"Crossing the Rubicon" means passing a point of no return. Where can you find the Rubicon today?

The meaning of the phrase "crossing the Rubicon" comes from an allusion to the crossing of the river Rubicon by Julius Caesar in early January 49 BCE. Caesar had been appointed to a governorship over a region that ranged from southern Gaul (the south of France today) to Illyricum (northeast Adriatic coast). As his term of governorship ended, the Senate ordered him to disband his army and return to Rome. It was illegal to bring armies into Italy and the Rubicon marked the boundary between the Roman province of Cisalpine Gaul and Italy proper, controlled directly by Rome. So, crossing the river under arms amounted to insurrection, treason, and a declaration of war on the state. Indeed, Caesar's crossing of the river precipitated civil war, which ultimately led to his becoming dictator for life.

The Romans referred to the river as “Rubico,” which comes from a Latin word meaning "red"; the waters are coloured red by iron deposits in the riverbed. Today it is the river Rubicone in the Emilia-Romagna region of northern Italy. It flows for around 80 km (50 miles) from the Apennine Mountains to the Adriatic Sea, just north of Rimini. The river was known as the Fiumicino until 1933, when it was identified with the ancient river Rubicon.

Seven years after Caesar's fateful crossing, the Rubicon ceased to be relevant: Octavian merged Cisalpine Gaul into Italy and the river ceased to be the extreme northern border of Italy. Shorn of its importance, the name gradually disappeared.