Final answer:
The assassination of Julius Caesar on March 15, 44 BCE, is traditionally considered the event that marked the end of the Roman Republic, leading to a civil war and the establishment of the Roman Empire.
Step-by-step explanation:
The event that marked the end of the Roman Republic was the assassination of Julius Caesar on the Ides of March in 44 BCE. Caesar was assassinated by a group of senators who believed they were acting to save the Republic from what they saw as his tyrannical rule. However, rather than restoring the Republic, Caesar's death led to a period of civil war, which ultimately resulted in the rise of his heir, Octavian (later known as Augustus), and the establishment of the Roman Empire. The era of Roman peace known as the Pax Romana began under Augustus. Although Caesar's crossing of the Rubicon in 49 BCE and his declaration as dictator for life were significant precursors to the end of the Republic, it was his assassination that is traditionally considered the definitive end.