The Rubicon River formed the border between Gaul and the Roman Republic. According to legend, when Caesar got to the river with his army, he still was not sure if he should cross and invade - something that would start a civil war.
In Suetonius' history of the emperors (written more than 100 years after Caesar crossed the Rubicon), he wrote: "Overtaking his cohorts at the river Rubicon, which was the boundary of his province, he paused for a while, and realizing what a step he was taking, he turned to those about him and said: 'Even yet we may draw back; but once cross yon little bridge, and the whole issue is with the sword.' As he stood in doubt, this sign was given him. On a sudden there appeared hard by a being of wondrous stature and beauty, who sat and played upon a reed; and when not only the shepherds flocked to hear him, but many of the soldiers left their posts, and among them some of the trumpeters, the apparition snatched a trumpet from one of them, rushed to the river, and sounding the war-note with mighty blast, strode to the opposite bank."
Then Caesar cried: 'Take we the course which the signs of the gods and the false dealing of our foes point out. Alea iacta est, ("the die is cast").
And so, the Roman Republic was thrown into a civil war that was eventually won by Caesar defeating Pompey and his allies.
The phrase "crossing the Rubicon" entered popular culture to mean "past the point of no return." A Google search on "crossing the Rubicon" will turn up everything from shopping sites to Vie-President Joe Biden saying "I crossed the Rubicon about not being president and being vice president when I decided to take this office" and Supreme Court justice Anthony Kennedy saying, "Sometimes you don't know if you're Caesar about to cross the Rubicon or Captain Queeg cutting your own tow line."