"Crossing the Rubicon" means embarking on a course of action on which there is no going back. The phrase comes from the time when Rome changed from a republic to an empire. Julius Caesar, who was the governor of the Roman province of Gaul, took his armies across the river Rubicon, which marked the boundary between Gaul and Italy, to attempt to seize power in Rome. The governor of a Roman province had great power within his own domain, including direct command over large armies. Because of this, Roman rulers had feared that a governor might be tempted to use his armies to seize power in the capital. For this reason, they made it a capital offence for a Roman governor to take his armies outside the boundaries of his province into Italian territory. So, when Caesar took his armies across the Rubicon, he had made himself liable to execution, and would either succeed in his goal of becoming Roman dictator or would fail and be killed.