Final answer:
The assassination of Julius Caesar marked the end of the Roman Republic and the beginning of the Roman Empire. Octavian, later known as Augustus Caesar, rose to power and established the empire.
Step-by-step explanation:
The event that marked the end of the Roman Republic and the beginning of the Roman Empire was the assassination of Julius Caesar on the Senate floor. This event took place on the Ides of March in 44 BCE. Following Caesar's death, a period of civil war ensued, which ultimately led to the rise of Octavian and the establishment of the Roman Empire. Under Octavian, who later became known as Augustus Caesar, a 200-year period of peace called the Pax Romana began.