Crossing the Rubicon

Crossing the Rubicon is a metaphor similar to "Burning one's bridges" and "the point of no return". It means passing a decisive point, where one cannot turn back. It comes from a genuine historical incident. The Rubicon is a shallow river in north-east Italy. According to Roman tradition and law Roman legions could not be brought south of the river without permission of the Roman Senate.
Caesar's civil war.
In 50 BC, the Senate, led by Pompey, ordered Julius Caesar to disband his army and return to Rome because his term as governor of Gaul had finished. Caesar thought he would be prosecuted if he entered Rome without the immunity enjoyed by a magistrate. Pompey accused Caesar of insubordination and treason.
Crossing the Rubicon.
Caesar and his army approached Rome and "crossed the Rubicon" in 49 BC. It was the point beyond which no army was supposed to go. The river marked the boundary between Cisalpine Gaul to the north, and Italy proper to the south. Pompey, the lawful Consul, and his friends, fled from Rome as Caesar's army approached. So began a civil war in which Caesar eventually triumphed.
What Caesar reportedly said at the crossing of the Rubicon is "Alea iacta est" ("The die is cast"). This is yet another metaphor. It comes from gambling with dice: once the die or dice have been thrown, no bets can be withdrawn or added, even before the dice have come to rest.