The Rubicon River was a border between the region of Gaul and the Italian peninsula during ancient Roman times. In 49 BC, Julius Caesar, then governor of Gaul, led his army across the Rubicon and into Italy, which marked the beginning of the Roman Civil War. Caesar emerged victorious, effectively ending the Roman Republic and ushering in the Roman Empire. Since then, the phrase "crossing the Rubicon" has been used to refer to a decisive step or irreversible decision.