http://en.wikipedia.org/wiki/RubiconThe Rubicon (Rubico, in Italian Rubicone) is an ancient Latin name for a small river in northern Italy. The river was considered to mark the boundary between the Roman province of Cisalpine Gaul to the north and the Roman heartland to the south;
An ancient Roman law forbade any general from crossing the Rubicon River and entering Italy proper with a standing army. To do so was treason.
The law thus protected the republic from internal military threat. This tiny stream would reveal Caesar's intentions and mark the point of no return.
When Julius Caesar crossed the Rubicon in 49 BC, supposedly on January 10 of the Roman calendar, in pursuit of Gnaeus Pompeius Magnus he broke that law and made armed conflict inevitable. According to Suetonius he uttered the famous phrase 'the die is cast'.1 Suetonius also described how Caesar was apparently still undecided as he approached the river, and the author gave credit for the actual moment of crossing to a supernatural apparition.
The phrase "crossing the Rubicon" has survived to refer to any person committing himself irrevocably to a risky course of action. It is also in limited usage as to its original meaning of using military power in the homeland.