Julius Caesar was the famous Roman general who crossed the Rubicon River in 49 BC. This act was significant as it plunged Rome into civil war and marked Caesar's defiance against the Roman Senate's authority. The phrase "crossing the Rubicon" has become synonymous with making irreversible decisions or committing to a course of action, emphasizing the risky nature of his choice. It ultimately led to Caesar's rise to power, the end of the Roman Republic, and the establishment of the Roman Empire, shaping the course of Western history.