Definition: The Rubicon was a stream separating Cisalpine Gaul from Italy. When Julius Caesar led his troops from Gaul to the Rubicon, Caesar paused on the northern end of the bridge debating whether to cross or not. It would be a crime against Rome for proconsul Caesar to bring his troops in from the province, but if he didn't, he would be stripped of command and prosecuted. Although he hesitated, Caesar did cross the Rubicon, in January 49 B.C., thereby starting a civil war. To cross the Rubicon means to take an irrevocable step that commits one to a specific course.