The expression means to make a difficult decision with irreversible consequences – in short, to pass the point of no return.
It refers back to a decision made by Julius Caesar in January 49 BC that changed Ancient Rome forever. As a successful governor of the Roman province of Gaul (modern-day France), many in Rome feared Caesar’s growing power so the Senate ordered him to disband his legions and return to Rome.
Caesar marched a single legion to the boundary between Gaul and Italy, marked by the small river, and he knew that to go any further was forbidden. In the eyes of Rome, he would be an enemy of the state but he still crossed the Rubicon, sparking civil war. It was at this moment that Caesar said the now famous phrase, “The die is cast.”
Having won the civil war – defeating the de facto leader of the Roman state, Pompey – Caesar named himself as the dictator of Rome. But it only lasted five years as he famously did not heed the warning of another famous idiom – “Beware the Ides of March” – and was stabbed to death.