In Antony and Cleopatra, at which naval battle is Antony's fleet defeated by Octavius?

In Antony and Cleopatra, Antony’s fleet is defeated by Octavius at the Battle of Actium.

The naval battle took place on 2 September 31 BC in the Ionian Sea near Actium, on the western coast of Greece. Octavius’s fleet was commanded at sea by Marcus Vipsanius Agrippa, while Mark Antony commanded the opposing forces and Cleopatra supplied a separate Egyptian squadron.

Antony’s ships were generally larger and heavily armed, but Octavius had more maneuverable vessels and benefited from better positioning and experienced crews. During the battle, Cleopatra’s squadron broke through the lines and withdrew. Antony followed her, while much of his fleet was left behind and defeated. The result ended Antony and Cleopatra’s immediate hopes of controlling the Roman world.

Actium is often confused with the later Battle of Alexandria in 30 BC. Alexandria was a subsequent land-and-sea campaign in Egypt, whereas Actium was the decisive naval engagement. Octavius’s victory eventually enabled him to become Augustus and marked the effective end of the Roman Republic.