On January 10th, in 49 B.C., Caesar led a legion of his troops across the Rubicon River and into Italy, igniting a civil war against Pompey. Roman Generals were forbidden to bring their troops into Italy, which is why him doing so was considered an act of war.
He is rumoured to have said “Alea iacta est” or “The die is cast”, when he did so, meaning that he had past the point of no return. Since he had done this, Caesar either had to triumph against Pompey or he would have been killed.
The phrase “Crossing the Rubicon” is still used today meaning someone has purposely passed the point of no return.