Octavian's victory enabled him to consolidate his power over Rome and its dominions. He adopted the title of Princeps ("first citizen") and some years later was awarded the title of Augustus ("revered") by the Roman Senate. This became the name by which he was known in later times. As Augustus, he retained the trappings of a restored Republican leader, but 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.
The alliance between Octavian, Mark Antony, and Marcus Lepidus, commonly known as the Second Triumvirate, was renewed for a five-year term in 38 BC. However, the triumvirate broke down when Octavian saw Caesarion, the professed son of Julius Caesar and Queen Cleopatra VII of Egypt, as a major threat to his power. This occurred when Mark Antony, the other most influential member of the triumvirate, abandoned his wife, Octavian's sister Octavia Minor. Afterwards he moved to Egypt to start a long-term romance with Cleopatra, becoming the de facto stepfather to Caesarion. Such an affair was doomed to become a political scandal. Antony was inevitably perceived by Octavian and the majority of the Roman Senate as the leader of a separatist movement that threatened to break the unity of the Roman Republic.
Octavian's prestige and, more importantly, the loyalty of his legions, had been initially boosted by Julius Caesar's legacy of 44 BC, by which 19-year-old Octavian was officially adopted as Caesar's only son and the sole legitimate heir of his enormous wealth. Antony had been the most important and most successful senior officer in Caesar's army (magister equitum) and, thanks to his military record, claimed a substantial share of the political support of Caesar's soldiers and veterans. Both Octavian and Antony had fought against their common enemies in the civil war that followed the assassination of Caesar.
After years of loyal cooperation with Octavian, Antony started to act independently, eventually raising his rival's suspicion that he was vying to become sole master of Rome. When he left Octavia Minor and moved to Alexandria to become Cleopatra's official partner, he led many Roman politicians to believe that he was trying to become the unchecked ruler of Egypt and of other eastern kingdoms, while still maintaining his command over the many Roman legions in the East. 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 and Cleopatra formally elevated Caesarion, then 13, to power in 34 BC, giving him the vague but alarming title of "King of the Kings" (Donations of Alexandria). Being a son of Caesar, such an entitlement was felt as a threat to Roman republican traditions. According to a widespread belief, Antony had once offered a diadem to Caesar. Thereafter, Octavian started a propaganda war, denouncing Antony as an enemy of Rome, asserting that he was seeking to establish a personal monarchy over the entire Roman Empire on the behalf of Caesarion, circumventing the Roman Senate. It was also said that Antony intended to move the capital of the empire to Alexandria.
As the Second Triumvirate formally expired on the last day of 33 BC, Antony wrote to the Senate that he did not wish to be reappointed. He hoped that he might be regarded by them as their champion against the ambition of Octavian, whom he presumed would not be willing to abandon his position in a similar manner. The causes of mutual dissatisfaction between the two had been continually accumulating. 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 for being 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 the acknowledgment of Caesarion as a legitimate son of Caesar were a degradation of his office and a menace to himself.
During 32 BC, a third of the Senate and both consuls allied with Antony. The consuls had determined to conceal the extent of Antony's demands. Gnaeus Ahenobarbus seems to have wished to keep quiet, but Gaius Sosius on 1 January 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. Octavian was not present, but at the next meeting made a reply of such a nature that the consuls both left Rome to join Antony; Antony, when he heard of it, after publicly divorcing Octavia, came at once to Ephesus with Cleopatra, where a vast fleet was gathered from all parts of the East, of which Cleopatra furnished a large proportion. After staying with his allies at Samos, Antony moved to Athens. His land forces, which had been in Armenia, came down to the coast of Asia, and embarked under L. Canidius Crassus.
Octavian was not behind in his strategic preparations. Military operations began in 31 BC, when Octavian's general Agrippa captured Methone, a Greek town allied to Antony. However, by the publication of Antony's will, which had been put into his hands by the traitor Plancus, and by carefully letting it be known in Rome what preparations were going on at Samos and how entirely Antony was acting as the agent of Cleopatra, Octavian produced such a violent outburst of feeling that he easily obtained Antony's deposition from the consulship of 31 BC, for which Antony had been designated. In addition to the deposition Octavian procured a vote for a proclamation of war against Cleopatra – well understood to mean against Antony, though he was not named. In doing this, the Senate issued a war declaration and deprived Antony of any legal authority.
Antony meant to anticipate an attack by a descent upon Italy towards the end of 32 BC, and went as far as Corcyra. But finding the sea guarded by a squadron of Octavian's ships, he retired to winter at Patrae while his fleet for the most part lay in the Ambracian Gulf, and his land forces encamped near the promontory of Actium, while the opposite side of the narrow strait into the Ambracian Gulf was protected by a tower and troops.
After Octavian's proposals for a conference with Antony had been scornfully rejected, both sides prepared for the struggle the next year. The early months passed without notable event, beyond some successes of Agrippa on the coasts of Greece, meant to divert Antony's attention. It was not until the latter part of August that troops were landed in the neighborhood of Antony's camp on the north side of the strait. Still Antony could not be tempted out. It took some months for his full strength to arrive from the various places in which his allies or his ships had wintered, and during these months not only was Agrippa continuing his descent upon Greek towns and coasts, but in various cavalry skirmishes Octavian had so far prevailed that Antony abandoned the north side of the strait and confined his soldiers to the southern camp. Cleopatra now earnestly advised that garrisons should be put into strong towns, and that the main fleet should return to Alexandria. 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.
Octavian learned of this and debated how to prevent it. At first of a mind to let Antony sail and then attack him, he was prevailed upon by Agrippa to give battle. On 1 September he issued an address to his fleet, preparing them for battle. The next day was wet and the sea was rough. 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. Octavian, after a short hesitation, ordered his vessels to steer to the right and pass the enemy's ships. For fear of being surrounded, Antony was forced to give the word to attack.
The two fleets met outside the Gulf of Actium (today Preveza), on the morning of 2 September 31 BC. Antony's fleet numbered 500, of which 230 were large war galleys with towers full of armed men. He led these through the straits towards the open sea. Octavian had about 250 warships. Octavian's fleet was waiting beyond the straits, led by the experienced admiral Agrippa, commanding from the left wing of the fleet, Lucius Arruntius the centre and Marcus Lurius the right. Titus Statilius Taurus commanded Octavian's armies, who observed the battle from shore to the north of the straits. Antony and Gellius Publicola commanded the right wing of the Antonian fleet, while Marcus Octavius and Marcus Insteius commanded the center, with Cleopatra's squadron positioned behind them. Gaius Sosius launched the initial attack from the left wing of the fleet, while Antony's chief lieutenant Publius Canidius Crassus was in command of the triumvir's land forces.
It is estimated that Antony had around 140 ships, vs 260 ships of Octavian's fleet. What Antony lacked in quantity was made up for in quality (of vessel), as his ships were mainly the standard Roman warship, quinqueremes with smaller quadriremes, heavier and wider than Octavian's and making them ideal weapon platforms. Antony's personal flag ship, like those of his admirals, was a "ten". An "eight" war galley had around 200 heavy marines, archers, and at least 6 ballista catapults. Being larger than Octavian's ships, Antony's war galleys were very difficult to board in close combat, and were able to rain down missiles onto their smaller and lower opponent's ships. The bows of the galleys were armored with bronze plates and square-cut timbers, making a successful ramming attack with similar equipment difficult. The only way to disable such a ship was to smash its oars, rendering it immobile and hopefully, isolated from the rest of its fleet. The main weakness of Antony's ships was their lack of maneuverability; such a ship, once isolated from support of its fleet, could be swamped with boarding attacks. However many of his ships were undermanned with rowing crews; there had been a severe malaria outbreak while they were waiting for Octavian's fleet to arrive.
Octavian's fleet was largely made up of smaller "Liburnian" vessels. Octavian's ships, although smaller, were still manageable in the heavy surf and could out-manoeuvre Antony's ships, get in close, attack the above-deck crew with a shower of arrows and ballista-launched stones, and then retreat. Moreover, his crews were better-trained, professional, well fed and rested. A medium ballista was capable of penetrating the sides of most warships at close range and had an effective range of around 200 yards. Most ballista firing was aimed at the marines on the fighting decks of the ships.
Before the battle one of Antony's generals, Quintus Dellius, had defected to Octavian, bringing with him Antony's battle plans.
Shortly after midday, Antony was forced to extend his line from the protection of the shore and finally engage the enemy. Seeing this Octavian's fleet put to sea. 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. By about noon the fleets were in formation but Octavian refused to be drawn out so Antony was forced to attack. The battle raged all afternoon without decisive result.
Cleopatra's fleet, in the rear, retreated to the open sea without engaging. A breeze sprang up in the right direction, and the Egyptian ships were soon hurrying out of sight.
Antony had not observed the signal, and believing that it was mere panic and all was lost, followed the flying squadron. The contagion spread fast; everywhere sails were seen unfurling, and towers and other heavy fighting gear going by the board. Some fought on, and it was not until long after nightfall, when many a ship was blazing from the firebrands thrown upon them, that the work was done. Making the best of the situation, he burned those ships he could no longer man, while clustering the remainder tightly together. With many oarsmen dead or unfit to serve, the powerful, head-on ramming tactic for which the Octaries had been designed was now impossible. 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. Those left behind were captured or sunk.
A differing account of the battle is argued by J.M. Carter. He postulates that Antony knew he was surrounded and had nowhere to run. To try to turn this to his advantage, Antony gathered his ships around him in a quasi-horseshoe formation, staying close to the shore for safety. Then, should Octavian's ships approach Antony's, the sea would push them into the shore. Antony foresaw that he would not be able to defeat Octavian's forces, so he and Cleopatra stayed in the rear of the formation. Eventually, Antony sent the ships on the northern part of the formation to attack. He had them move out to the north, spreading out Octavian's ships which up until now were tightly arranged. He sent Gaius Sosius down to the south to spread the remaining ships out to the south. This left a hole in the middle of Octavian's formation. Antony seized the opportunity and with Cleopatra on her ship and him on a different ship, sped through the gap and escaped, abandoning his entire force.
With the end of the battle, Octavian exerted himself to save the crews of the burning vessels, and had to spend the whole night on board. On the next day, as much of the land army as had not escaped to their own lands submitted or were followed in their retreat to Macedonia and forced to surrender, and Antony's camp was occupied, bringing an end to the war.
The political consequences were far-reaching. Under cover of darkness some 19 legions and 12,000 cavalry fled before Antony was able to engage Octavian in a land battle. Thus, after Antony lost his fleet, his army, which had been equal to that of Octavian, deserted. Antony, though he had not laid down his imperium, was a fugitive and a rebel without that shadow of a legal position which the presence of the consuls and senators had given him in the previous year. Some of the victorious fleet went in pursuit of him; but Octavian himself visited Greece and Asia, and spent the winter at Samos; though he was obliged to go for a short time to Brundisium to settle a mutiny and arrange for assignations of land.
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. She was allowed to believe that she would be well treated, for Octavian was anxious to secure her for his triumph. Antony, who had found himself generally deserted, after vainly attempting to secure the army stationed near Paraetonium under Pinarius and sending his eldest son Antyllus with money to Octavian and an offer to live at Athens as a private citizen, found himself in the spring attacked on two sides. C. Cornelius Gallus was advancing from Paraetonium; and Octavian landed at Pelusium, with the connivance it was believed of Cleopatra. Antony was defeated by Gallus, and returning to Egypt, advanced on Pelusium.
Despite a victory at Alexandria on 31 July 30 BC, more of Antony's men deserted, leaving him with insufficient forces to fight Octavian. A slight success over Octavian's tired soldiers encouraged him to make a general attack, in which he was decisively beaten. Failing to escape on board a ship, he stabbed himself upon mistakenly believing that Cleopatra had already died. He did not die at once, and when he found out that Cleopatra was still alive, he insisted on being taken to the mausoleum in which Cleopatra was hiding, and died in her arms. She was shortly afterwards brought to the palace and vainly attempted to move Octavian to pity.
Cleopatra committed suicide, on 12 August 30 BC. In one account, she put an end to her life by the bite of an asp conveyed to her in a basket of figs. Octavian had Caesarion killed later that month, finally securing his legacy as Caesar's only 'son'.
Octavian's victory at Actium gave him sole and uncontested control of "Mare Nostrum" (Our Sea, i.e., the Roman Mediterranean) and he became "Augustus Caesar" and the "first citizen" of Rome. This victory, consolidating his power over every Roman institution, marked the transition of Rome from Republic to Empire. Egypt's surrender following Cleopatra's death marked the final demise of both the Hellenistic Period and the Ptolemaic Kingdom.