What does Crossing the Rubicon mean?

Answer 1

"Crossing the Rubicon" is a popular idiom meaning to pass a point of no return. It refers to Caesar's 49 BC crossing of the river, which was considered an act of war. (direct quote from wikipedia's "rubicon" page)

Answer 2

Crossing the Rubicon is an expression meaning "no turning back." A similar expression is "burning your bridges." It means that once you do it, you have taken an irrevocable, probably risky step and understand that you must accept the consequences whatever they are.

The reference to the Rubicon goes to Julius Caesar in 49 BCE. Roman law then prohibited any Roman Army legion from crossing the Rubicon, a river that marked the northern boundary between Gaul and Italy. The law was to protect Rome from internal military threat. Caesar had been ordered to return to Rome by his arch-rival Pompey, but Caesar knew that if he did so without an army at his back he would probably lose all the power he'd built up, so he took the irrevocable step of illegally crossing the Rubicon with a legion and ignited a civil war. Another expression of inevitability that comes from this same act is, "The die is cast," which is what Caesar reportedly said when he ordered his troops over the river. That refers to throwing dice, as in taking a gamble. "Die" is the singular of "dice."