As [Julius Caesar] stood, in two minds, an apparition of superhuman size and beauty was seen sitting on the river bank playing a reed pipe. A party of shepherds gathered around to listen and, when some of Caesar's men, including some of the trumpeters, broke ranks to do the same, the apparition snatched a trumpet from one of them, ran down to the river, blew a thunderous blast, and crossed over. Caesar exclaimed: 'Let us accept this as a sign from the Gods, and follow where they beckon, in vengeance on our double-dealing enemies. The die is cast.'
This, of course, is the famous crossing of the River Rubicon, the historical border of Rome beyond which generals could not take their standing armies on pain of death. By crossing this boundary with his soldiers, Julius Caesar was committing an act of treason that was fully punishable by Roman law, and the only way to escape execution was for Caesar to depose the current government and seize control for himself. It was, quite literally, a point of no return.
Famously, Caesar did cross the Rubicon (apparently at the urging of an impetuous, musical ghost). He won Rome, set up an empire, proclaimed himself perpetual dictator, and became a force that would shape the development of Western civilization. And, in the process, he also coined a succinct and useful phrase that thrives even today.