"Crossing the Rubicon" means passing a point of no return.

The expression is an alusion to a pivotal moment in History where (at the time) regional governer Julius Caeser committed a capital crime by entering with his army in Italian territory (by crossing the river Rubicon) which was heavily forbidden by the Senate.

After some indecision, he crossed with his army in open defiance and knowing it would cause a civil war. We would eventually win the Civil War and became the first de facto Emperor, laying the transition from the Roman Republic to the Roman Empire.

By deciding to cross that river, he changed the course of History in an irreversible way. On the night of the crossing, it is reported he uttered the famous quote "Alea iacta est" which means "the ink has been cast".

Good or bad, he was not a fence sitter. Profound transformations often require unwavering irreversible commitment.