The battle took place on 2 September 31 BC in the Ionian Sea, near the former Roman colony of Actium, Greece, and was the climax of over a decade of rivalry between Octavian and Antony.

Octavian previously gained a preliminary victory in Greece, where his navy successfully ferried troops across the Adriatic Sea under the command of Agrippa.

[4] Antony's fleet sailed through the bay of Actium on the western coast of Greece, in a desperate attempt to break free of the naval blockade.

[7] The alliance among Octavian, Mark Antony and Lepidus, commonly known as the Second Triumvirate, was renewed for a five-year term at Tarentum in 37 BC.

[8] However, the triumvirate broke down when Octavian saw Caesarion, the professed son of Julius Caesar[9] and Queen Cleopatra VII of Egypt, as a major threat to his power.

[10] This occurred when Mark Antony, the other most influential member of the triumvirate, abandoned his wife, Octavian's sister Octavia Minor.

After years of loyal cooperation with Octavian, Antony started to act independently, eventually arousing his rival's suspicion that he was vying to become sole master of Rome.

[citation needed] As a personal challenge to Octavian's prestige, Antony tried to get Caesarion accepted as a true heir of Caesar, even though the legacy did not mention him.

Antony complained that Octavian had exceeded his powers in deposing Lepidus, in taking over the countries held by Sextus Pompeius and in enlisting soldiers for himself without sending half to him.

Octavian complained that Antony had no authority to be in Egypt; that his execution of Sextus Pompeius was illegal; that his treachery to the king of Armenia disgraced the Roman name; that he had not sent half the proceeds of the spoils to Rome according to his agreement; and that his connection with Cleopatra and acknowledgement of Caesarion as a legitimate son of Caesar were a degradation of his office and a menace to himself.

Ahenobarbus seems to have wished to keep quiet, but on 1 January Sosius made an elaborate speech in favor of Antony, and would have proposed the confirmation of his act had it not been vetoed by a tribune.

The early months passed without any notable events, other than some successful forays by Agrippa along the coasts of Greece, primarily designed to divert Antony's attention.

During these months Agrippa continued his attacks upon Greek towns along the coast, while Octavian's forces engaged in various successful cavalry skirmishes, so that Antony abandoned the strait's north side between the Ambracian Gulf and the Ionian Sea and confined his soldiers to the southern camp.

The large contingent furnished by Egypt gave her advice as much weight as her personal influence over Antony, and it appears that this movement was agreed to.

When the trumpet signal for the start rang out, Antony's fleet began issuing from the straits and the ships moved into line and remained quiet.

Sosius launched the initial attack from the fleet's left wing while Antony's chief lieutenant Publius Canidius Crassus commanded the triumvir's land forces.

[20] Pelling notes that the presence of two former consuls on Antony's side commanding the wings indicates that it was there that the major action was expected to take place.

What Antony lacked in quantity was made up for in quality: his ships were mainly the standard Roman warship, quinqueremes with smaller quadriremes, heavier and wider than Octavian's, making them ideal weapon platforms.

The galleys' bows were armoured with bronze plates and square-cut timbers, making a successful ramming attack with similar equipment difficult.

Antony had hoped to use his biggest ships to drive back Agrippa's wing on the north end of his line, but Octavian's entire fleet, aware of this strategy, stayed out of range.

Antony transferred to a smaller vessel with his flag and managed to escape, taking a few ships with him as an escort to help break through Octavian's lines.

Though he had not laid down his imperium, Antony was a fugitive and a rebel without that shadow of a legal position the presence of the consuls and senators had given him in the previous year.

[16] At Samos Octavian received a message from Cleopatra with the present of a gold crown and throne, offering to abdicate in favor of her sons.

Despite a minor victory at Alexandria on 31 July 30 BC, more of Antony's men deserted, leaving him with insufficient forces to fight Octavian.

Failing to escape by ship, he stabbed himself in the stomach upon mistakenly believing false rumours propagated by Cleopatra claiming that she had committed suicide.

Octavian had previously shown little mercy to surrendered enemies and acted in ways that had proven unpopular with the Roman people, yet he was given credit for pardoning many of his opponents after the Battle of Actium.

Octavian's victory at Actium gave him sole, uncontested control of "Mare Nostrum" ("Our Sea", i.e., the Roman Mediterranean) and he became "Augustus Caesar" and the "first citizen" of Rome.

Egypt's surrender after Cleopatra's death marked the demise of both the Hellenistic Period and the Ptolemaic Kingdom,[37] turning it into a Roman province.

On a hill just north of the newly founded Nicopolis, at the site where he had made his camp in 31 BC, he constructed the campsite memorial, a tropaion decorated with the bronze rostra (rams) taken from the captured warships of Antony's fleet.

[39] At the cape of Actium, Octavian built the Actian dekanaia, another tropaion composed of the dedication a whole set of ten warships captured from his enemy's fleet.