(September 2, 31 BC), naval battle off a promontory in the north of Acarnania, on the western coast of Greece, where Octavian (known as the emperor Augustus after 27 BC), by his decisive victory over Mark Antony (Antony, Mark), became the undisputed master of the Roman world. Antony, with 500 ships and 70,000 infantry, made his camp at Actium, which lies on the southern side of a strait leading from the Ionian Sea into the Ambracian Gulf. Octavian, with 400 ships and 80,000 infantry, arrived from the north and, by occupying Patrae and Corinth, also managed to cut Antony's southward communications with Egypt via the Peloponnese.

Desertions by some of his allies and a lack of provisions soon forced Antony to take action. Either hoping to win at sea because he was outmaneuvered on land or else simply trying to break the blockade, Antony followed Cleopatra's advice to employ the fleet. He drew up his ships outside the bay, facing west, with Cleopatra's squadron behind. The ensuing naval battle was hotly contested, with each side's squadrons trying to outflank the other, until Cleopatra took her Egyptian galleys and fled the battle. Antony then broke off and with a few ships managed to follow her. The remainder of his fleet became disheartened and surrendered to Octavian, and Antony's land forces surrendered one week later.