Rubicon

In 49 B.C., Julius Caesar led his army to the banks of the Rubicon, a small river that marked the boundary between Italy and Gaul. Caesar knew Roman law forbade a general from leading his army out of the province to which he was assigned. By crossing the Rubicon, he would violate that law. "The die is cast," he said, wading in. That act of defiance sparked a three-year civil war that ultimately left Julius Caesar the undisputed ruler of the Roman world. It also inspired English speakers to adopt two popular sayings -crossing the Rubicon and the die is cast-centuries later. Rubicon has been used in English as the name of a significant figurative boundary since at least the early 1600s.