With five hundred ships and seventy thousand infantry, Antony makes his camp at Actium, which lies on the southern side of a strait leading from the Ionian Sea into the Ambracian Gulf.

Octavian, with four hundred ships and eighty thousand infantry, arrives from the north and, by occupying Patrae and Corinth, manages to cut Antony's southward communications with Egypt via the Peloponnese.

Antony's army is trapped on land and sea. Deserters flee to Octavian's side daily while Octavian's forces are comfortable enough to make preparations.

Many of the soldiers, disgusted with Antony’s conduct, offer the command to the consul Ahenobarbus, but he prefers to desert the party altogether, and defects to Octavian shortly before the Battle of Actium.

Suffering from a fever, he takes a small boat to Octavian's side.

Antony is greatly upset by this defection, but he nevertheless sends Ahenobarbus him all his gear, his friends and his attendants.

Ahenobarbus is not, however, present at the battle itself, as he dies a few days after joining Augustus.

Plutarch suggests that his death was due to "the shame of his disloyalty and treachery being exposed."

Suetonius says that he was the best of his family.

Desertions by some of his allies and a lack of provisions soon force Antony on September 2, 31 BCE to take action.

Either hoping to win at sea because he is outmaneuvered on land or making a desperate attempt to break free of the naval blockade, he draws up his ships outside the bay, facing west, with Cleopatra's squadron behind.

Sailing his fleet through the bay of Actium on the western coast of Greece, Antony fleet faces the much larger fleet of smaller, more maneuverable ships under commanders Agrippa and Gaius Sosius.

In the ensuing naval contest, each side's squadrons try to outflank the other until Cleopatra, by prearranged plan rather than treachery, takes her sixty Egyptian galleys (carrying her and Antony's treasury) and flees the battle.

Antony then breaks off and with a few ships manages to follow her.

The remainder of his fleet becomes disheartened and surrenders to Octavian; Antony's land forces surrender one week later.

Octavian has previously shown little mercy to military combatants and acted in ways that have proven unpopular with the Roman people, yet he is given credit for pardoning many of his opponents after the Battle of Actium.