The event that signified the end of the Roman Republic and the rise of the Roman Empire was the assassination of Julius Caesar in 44 BCE. This pivotal act contributed to a power struggle and a series of civil wars, ultimately leading to the rise of Caesar Augustus as the first Roman emperor. The transition from a republic to an empire marked a significant shift in governance, consolidating power in a single ruler and altering the political landscape of ancient Rome, which had profound effects on Western civilization.