Caesar was killed on Mar. 15, 44 B.C. His assassination was carried out by a conspiracy of senators who sought to remove him from power.
The seeds of his downfall were sown several years earlier when he let his ambition get the better of him and marched his army into Rome. He defied a law that was meant to protect Rome from military conquest by setting the Rubicon as boundary which armies were prohibited to cross.
It was such a pivotal event in history that it became a figure of speech to say that someone had "crossed the Rubicon," meaning they had passed a point of no return.