«Ἀνερρίφθω κύβος» (anerriphtho kybos, lit. Let the die be cast) was attributed by Suetonius to Caesar when it was reported that some legionaries cross the Rubicon. Why did Caesar move to the Rubicon and stop there? Was it because after that, a civil war would be unavoidable and Caesar wanted to avoid it? Was it just an excuse to start the war anyway?
The Rubicon river marked the boundary between the province of Cisalpine Gaul and Italy proper. Caesar, as a proconsul, held imperium (the right to command) within the provinces, but only a consul or praetor could hold imperium inside Italy. Generals were expected to lay down their command and re-enter Italy as private citizens; not doing so would be seen as a threat to Rome. According to Wikipedia, "Suetonius's account depicts Caesar as undecided as he approached the river, and attributes the crossing to a supernatural apparition", suggesting that he was unsure whether to provoke civil war at that time. His act of crossing the Rubicon leading fully armed soldiers immediately created a force in Italy in opposition to the Senate; thus, a civil war had begun.
Wikipedia has a much more detailed (and better cited!) section on this very subject.