The famous military leader who crossed the Rubicon was Julius Caesar. In 49 BC, Caesar's decision to cross this boundary, which was forbidden by Roman law, marked a pivotal moment in Roman history, leading to the Roman Civil War. This act symbolized a point of no return and the assertion of his power against the Senate. Caesar’s gamble ultimately led to his rise as a dictator and the transformation of the Roman Republic into the Roman Empire, significantly reshaping the course of ancient history and governance.