from Wikipedia:
“As his term of governorship ended, the Roman Senate ordered Caesar to disband his army and return to Rome. He was explicitly ordered not to take his army across the Rubicon river, which was at that time a northern boundary of Italy. On January 10th of 49 BC, Caesar brought the 13th legion across the river, which the Roman government considered insurrection, treason, and a declaration of war on the Roman Senate. According to some authors, he is said to have uttered the phrase “alea iacta est”—the die is cast—as his army marched through the shallow river.
Today, the phrase “crossing the Rubicon” is a metaphor that means to pass a point of no return.”
Jeff Nirenberg 