The pivotal event that marked the transition from the Roman Republic to the Roman Empire was Julius Caesar's crossing of the Rubicon River in 49 BCE. This act symbolized a declaration of civil war against the Roman Senate and was a decisive moment that undermined the republican system of governance. Following Caesar's victory over Pompey and the Senate, he became the dictator of Rome, a position that disrupted the traditional power balances. His assassination in 44 BCE and the subsequent rise of his adopted heir, Octavian (later Augustus), culminated in the establishment of the Roman Empire, effectively ending the Roman Republic.