From Wikipedia
The Battle of Actium was the decisive confrontation of the Final War of the Roman Republic. It was fought between the forces of Octavian on the one side and the combined forces of Mark Antony and Cleopatra on the other. The battle took place on September 2, 31 BC, on the Ionian Sea near the Roman colony of Actium in Greece. Octavian's fleet was commanded by Marcus Vipsanius Agrippa, while Antony's fleet was supported by the ships of his beloved, Cleopatra VII, Queen of Ptolemaic Egypt.
Octavian's victory enabled him to consolidate his power over Rome and its dominions. To that end, he adopted the title of Princeps ("first citizen") and as a result of the victory was awarded the title of Augustus by the Roman Senate. As Augustus, he would retain the trappings of a restored Republican leader; however, historians generally view this consolidation of power and the adoption of these honorifics as the end of the Roman Republic and the beginning of the Roman Empire.
