The event that symbolized a turning point in Roman history and marked the beginning of the Roman Empire was the Battle of Actium in 31 BCE. This naval battle saw the forces of Octavian defeating those of Mark Antony and Cleopatra. Following this victory, Octavian returned to Rome and was granted the title Augustus, effectively establishing the principate and transitioning Rome from a republic to an empire. This change in governance led to an era of relative peace known as the Pax Romana, which lasted for over two centuries.