Julius Caesar’s time was up as the General in charge of Cisalpine Gaul, a province of the Roman Republic encompassing current France, etc.
The Senate had ordered Caesar home to Rome…and by tradition, to leave his army behind; Armies were not allowed in the Republic proper, and for good reason.
As Caesar sat on the other side of the river Rubicon, with his army, he made a decision to cross. Once he did so he would become a criminal, committing an act of war. And there would be no turning back.
He made his decision and plunged Rome into a civil war. He would be named dictator for life when he was victorious, something that was as significant for Romans as it would be for us.
Hence, “crossing the Rubicon” and “the die is cast” are synonymous with making an irreversible decision.