The Rubicon River was a river in northern Italy in Roman times that was used as a boundary under Roman law. No leader was allowed to bring an army into Italy, which meant passing the Rubicon. As the flow of rivers change over time, modern geologists can only approximate where the river was located during the time of Ancient Rome.
Crossing the Rubicon
Crossing the Rubicon is an expression that refers to Julius Caesar's decision in 49 B.C. to cross the Rubicon River with an army as he marched toward the Roman Senate and therefore war. After his great success in Gaul, forces allied against him had the Senate pass a decree that he had to give up his armies and return to Rome - most likely to be brought up on charges. Caesar defied them. The expression means to go beyond the point of no return, much as Caesar did when he crossed the Rubicon River. At the time that he crossed he uttered his famous phrase, "Alea iacta est!" ("The die is cast").