Rise of Augustus

Growing tensions

Arriving in Rome on 6 May 44 BC, Octavian found consul Mark Antony, Caesar's former colleague, in an uneasy truce with the dictator's assassins. They had been granted a general amnesty on 17 March in an agreement that they would respect the magistracies installed and laws passed by Caesar to avoid the political turmoil of invalidating them. Soon afterwards, Antony succeeded in driving most of them out of Rome with an inflammatory eulogy at Caesar's funeral, mounting public opinion against the assassins.

Mark Antony was amassing political support, but Octavian still had the opportunity to rival him as the leading member of the faction supporting Caesar. Antony had lost the support of many Romans and supporters of Caesar when he initially opposed the motion to elevate Caesar to divine status. Antony refused to hand over the money due Octavian as Caesar's heir, possibly on grounds that it would take time to disentangle it from state funds, but also as a measure to delay Octavian from carrying out the popular provision in Caesar's will that promised the dispersal of 300 sesterces per capita to the urban plebs of Rome. As consul, Antony blocked the curiate assembly from hearing Octavian's petition to legitimize his supposed adoption by Caesar, Octavian's attempts to reinstate Caesar's golden throne for public view at games staged in April and June, and Octavian's attempts to have Caesar formally deified after a comet seen in July during games honoring Caesar (and Venus) was widely interpreted as a sign of his divinity. Despite Antony's obstructionist tactics, during Caesar's victory games Octavian was able to distribute some of the funds in Caesar's will and combine this with other personal finances to cover half of the promised payouts to the plebs, enhancing his popularity while damaging that of Antony.

During the summer of 44 BC, Octavian won the support of Caesarian veterans and also made common cause with those senators—many of whom were themselves former Caesarians—who perceived Antony as a threat to the state. Antony had lictors drag Octavian away from a trial hearing over the reinstatement of private property seized by Caesar in 49 BC, after which Octavian claimed Antony threatened his life as retribution for ensuring the plebs received their rightful dues. Caesar's veterans then convinced Antony to publicly reconcile with Octavian in the Temple of Jupiter Optimus Maximus. Following this abortive attempt at reconciliation, Antony's bellicose edicts against Brutus and Cassius alienated him from the moderate Caesarians in the Senate, who feared a renewed civil war. In September, Marcus Tullius Cicero, now a political ally of Octavian, began to attack Antony in a series of speeches portraying him as a threat to the republican order.

First conflict with Antony

With opinion in Rome turning against him and his year of consular power nearing its end, Antony attempted to pass a plebiscite that would assign him proconsular governorship over the strategic province of Cisalpine Gaul in northern Italy, rather than the province of Macedonia. Octavian meanwhile built up a private army in Italy by recruiting Caesarian veterans, and in early November entered Rome with this private force to challenge Antony. However, they vacated the city shortly afterwards, due to some veterans choosing to quit once it became clear they were involved in a Caesarian squabble rather than a revenge campaign against Caesar's assassins. Nevertheless, on 28 November Octavian won over two of Antony's legions with the enticing offer of monetary gain.

In the face of Octavian's large and capable force, Antony saw the danger of staying in Rome and, to the relief of the Senate, he left Rome for Cisalpine Gaul, which was to be handed to him on 1 January 43 BC. However, the province had earlier been assigned to Decimus Junius Brutus Albinus, one of Caesar's assassins, who now refused to yield to Antony. Antony besieged him at Mutina, and rejected the resolutions passed by the Senate to stop the fighting. The Senate had no army to enforce their resolutions. This provided an opportunity for Octavian, who was already known to have armed forces. Cicero also defended Octavian against Antony's taunts about Octavian's lack of noble lineage and aping of Julius Caesar's name. In defense of Octavian against the taunts of Mark Antony, Cicero stated "we have no more brilliant example of traditional piety among our youth".

At the urging of Cicero, the Senate inducted Octavian as senator on 1 January 43 BC, yet he also was given the power to vote alongside the former consuls and the ability to stand for election of offices at an earlier age than usual. In addition, Octavian was granted imperium pro praetore, which legitimized his command, and was sent to relieve the siege along with Hirtius and Pansa (the consuls for 43 BC). He assumed the fasces on 7 January, a date that he would later commemorate as the beginning of his public career. Antony's forces were defeated at the battles of Forum Gallorum (14 April) and Mutina (21 April), forcing Antony to retreat to Transalpine Gaul. Both consuls were killed, however, leaving Octavian in sole command of their armies. These victories earned him his first acclamation as imperator, a title reserved for victorious commanders.

The Senate heaped many more rewards on Decimus Brutus than on Octavian for defeating Antony, then attempted to give command of the consular legions to Decimus Brutus. In response, Octavian stayed in the Po Valley and refused to aid any further offensive against Antony. In July, an embassy of centurions sent by Octavian entered Rome and demanded the consulship left vacant by Hirtius and Pansa, with Cicero as the proposed co-consul, and also that the decree should be rescinded which declared Antony a public enemy. When this was refused, he marched on the city with eight legions. He encountered no military opposition in Rome and on 19 August 43 BC was elected consul with his relative Quintus Pedius as co-consul. At age 19, Octavian became the youngest Roman to be elected as consul, and while the election was maintained with some formality, it was conducted with Octavian's forces stationed on the Campus Martius.

Pedius passed legislation that created a special tribunal; over a single day with Octavian as president, the tribunal tried, convicted, and passed sentences of exile on the Caesarian assassins and alleged associates in absentia. Octavian also induced the curiate assembly to have him adrogated into Caesar's family, sanctioning his legally dubious claim of testamentary adoption. Meanwhile, Antony formed an alliance with Marcus Aemilius Lepidus, then governor of Gallia Narbonensis. A fellow Caesarian, Lepidus was branded by the Senate as a public enemy for joining Antony. While Octavian marched north to fight Decimus Brutus and meet with Antony, Pedius convinced the Senate to revoke the law branding Antony and Lepidus as outlaws. Octavian's mother Atia died at some point before he officially allied with Antony and Lepidus in November, with nothing known about her death except that, as consul, Octavian arranged a lavish public funeral service for her.

Second Triumvirate

In a meeting near Bononia in October 43 BC, Octavian, Antony, and Lepidus sketched plans to form the triumvirate, ostensibly for the putting together of the republic. Their agreement, legitimized by law for five years, was then put in to force by the lex Titia, passed by tribune Publius Titius on 27 November that year. The triumvirate, unlike the unofficial "first triumvirate" of Caesar, Pompey, and Crassus, was a formal office; it gave the three men consular power, the right to appoint the magistrates, and allowed their division among themselves of the provinces not under the control of the liberatores in the east. Octavian had previously been engaged to Servilia, daughter of Publius Servilius Isauricus, but instead became engaged to Claudia, stepdaughter of Antony, a marriage alliance that was intended to solidify their official political union. For the remainder of 43 BC Octavian also relinquished the consulship to Antony's political ally Publius Ventidius, while Quintus Pedius, who died of unknown causes during the proscriptions, was replaced by Gaius Carrinus, another ally belonging to Antony. Allied Caesarian consuls for the ensuing years 42, 41, and 40 BC were also designated at this time, while most magisterial positions in Rome and western provinces were filled with men appointed by the triumvirs (the eastern provinces were still largely controlled by the liberators, or assassins of Caesar).

Proscriptions

The triumvirs then set in motion proscriptions, in which some 300 men were targeted as outlaws, divided roughly evenly between senators and equestrians. Thousands more had their properties confiscated. Contemporary Roman historians provide conflicting reports as to which triumvir was most responsible for the proscriptions and killing. However, the sources agree that enacting the proscriptions was a means by all three factions to eliminate political enemies.

Roman historian Velleius Paterculus asserted that Octavian tried to avoid proscribing officials whereas Lepidus and Antony were to blame for initiating them. Cassius Dio defended Octavian as trying to spare as many as possible, whereas Antony and Lepidus, being older and involved in politics longer, had many more enemies to deal with. This claim was rejected by Appian, who maintained that Octavian shared an equal interest with Lepidus and Antony in eradicating his enemies. Suetonius said that Octavian was reluctant at first to proscribe officials but did pursue his enemies with more vigor than the other triumvirs. Plutarch described the proscriptions as a ruthless and cutthroat swapping of friends and family among Antony, Lepidus, and Octavian. For example, Octavian allowed the proscription of his ally Cicero, Antony the proscription of his maternal uncle Lucius Julius Caesar (the consul for 64 BC), and Lepidus his brother Paullus. Plutarch insisted that Octavian defended Cicero at first before giving in to Antony's bloodlust, but historian Patricia Southern suggests that Octavian had his own motives for allowing Cicero to be killed. Southern writes that the "unpalatable truth" is that Octavian had a "vested interest" in removing political enemies, and that Cicero "would perhaps in turn have eliminated [Octavian] if it had been necessary".

The proscriptions were motivated in part by a need to raise money to pay the salaries of their troops for the upcoming conflict against Caesar's assassins, Marcus Junius Brutus and Gaius Cassius Longinus, but the main intention was the removal of wartime rivals. Rewards for their arrest gave incentive for Romans to capture those proscribed, while property of those arrested were seized by the triumvirs. However much money was raised was insufficient, possibly due to the few bidders for properties of proscript victims. The triumvirs then introduced a range of new taxes to fund their war. They reinstituted property taxes (in abeyance since 167 BC) and created new imposts on slaves, before also demanding property assessments for taxes on rich women that were reduced after a public protest of women in Rome.

Battle of Philippi and division of territory

On 1 January 42 BC, with Lepidus as consul, the Senate posthumously recognized Julius Caesar as a divinity of the Roman state, divus Iulius. Octavian was able to further his cause by emphasizing the fact that he was divi filius ('Son of the Divine'). Antony and Octavian then sent twenty-eight legions by sea to face the armies of Brutus and Cassius, who had built their base of power in Greece and eastern provinces. After two battles at Philippi in Macedonia in October 42 BC, the Caesarian army was victorious and Brutus and Cassius committed suicide.

Mark Antony used the battles of Philippi as a means to belittle Octavian, as both engagements were decisively won with the use of Antony's forces. In addition to claiming responsibility for both victories, Antony branded Octavian as a coward for handing over his direct military control to Marcus Vipsanius Agrippa instead. Octavian was bedridden with illness during the first battle, allegedly removing himself from command over the camp per his doctor's advice, but captured Brutus' camp during the second battle. Galinsky asserts that Octavian saved face from this embarrassment by having the corpse of Brutus beheaded, though historian Adrian Goldsworthy says it is unclear who gave this order. The remains were sent back to Rome for public display, but these were lost in a storm at sea before they could arrive.

After Philippi, a new territorial arrangement was made among the members of the triumvirate. Lepidus was suspected by Octavian and Antony of colluding with Sextus Pompeius, the son of Pompey and renegade general who had been given command over all Mediterranean coastlines by the anti-Caesarian Senate in 43 BC. Ratified on 20 March 43 BC, the Senate bestowed Sextus Pompeius with the office of praefectus classis et orae maritimae for the Roman navy, granting him control over all coastal areas of the Mediterranean Sea. Cisalpine Gaul was combined with Italia and given to Octavian along with the provinces of Hispania Citerior and Hispania Ulterior that Lepidus had to forfeit. Antony travelled east to Egypt where he allied himself with Queen Cleopatra, a Roman client ruler, former lover of Julius Caesar, and mother of Caesar's son Caesarion. In addition to eastern provinces, Antony also took Gallia Narbonensis from Lepidus, and already controlled Gallia Comata. Lepidus was left with the province of Africa, stymied by Antony, who conceded Hispania to Octavian.

Octavian was left to decide where in Italy to settle tens of thousands of veterans of the African and Macedonian campaigns, whom the triumvirs had promised to discharge. Those who had fought on the republican side with Brutus and Cassius could easily ally with a political opponent of Octavian if not appeased, and they also required land. There was no more government-controlled land to allot as settlements for their soldiers, so Octavian chose to alienate many Roman citizens by confiscating their land, instead of alienating many Roman soldiers who could mount a considerable opposition against him in the Roman heartland. There were as many as eighteen Roman towns affected by the new settlements, with entire populations driven out or at least given partial evictions.

The Perusine War, marriage alliances, and Treaty of Brundisium

There was widespread dissatisfaction with Octavian over these settlements of his soldiers, and this encouraged many to rally at the side of Lucius Antonius, who was brother of Mark Antony and supported by a majority in the Senate. Meanwhile, Octavian asked for a divorce from Claudia, the daughter of Antony's wife Fulvia and her first husband Publius Clodius Pulcher. He returned Claudia to her mother, claiming that their marriage had never been consummated. Fulvia decided to take action. Together with Lucius Antonius, she raised an army in Italy to fight for Antony's rights against Octavian, and Lucius Antonius even briefly took Rome, forcing Lepidus and his two legions to flee the city. Yet Lucius and Fulvia took a political and martial gamble in opposing Octavian, since the Roman army still depended on the triumvirs for their salaries. Lucius and his allies ended up in a defensive siege at Perusia, where Octavian forced them into surrender in February 40 BC.

Lucius and his army were spared because of his kinship with Antony, the strongman of the East, while Fulvia fled in exile to Sicyon in Greece. She died shortly afterwards, with blame for the revolt conveniently placed on her rather than on Lucius. Octavian showed no mercy, however, for the mass of allies loyal to Lucius. On 15 March, the anniversary of Julius Caesar's assassination, he had 300 Roman senators and equestrians executed for allying with Lucius. Perusia was also pillaged and burned, though it is unclear if Octavian's troops or local inhabitants started the fires. This bloody event sullied Octavian's reputation and was criticized by many, such as Augustan poet Sextus Propertius.

Sextus Pompeius affirmed his control of his powerbase in Sicily as part of an agreement reached with the triumvirate in 40 BC, and gained control of Sardinia and Corsica in 39 BC. Both Antony and Octavian were vying for an alliance with Pompeius. Octavian succeeded in a temporary alliance in 40 BC when he married Scribonia, a sister (or daughter) of Pompeius's father-in-law Lucius Scribonius Libo (hence an aunt of Sextus Pompey's wife). Scribonia gave birth to Octavian's only natural child, Julia, the same day that he divorced her to marry Livia Drusilla, little more than a year after their marriage. When Livia began her affair with Octavian, she was already married to Tiberius Claudius Nero, had a son Tiberius with Nero, and was pregnant with their second child. She gave birth to her second son, Drusus, several months after her divorce from Nero and marriage to Octavian. Livia later had another child with Octavian, who was born prematurely and did not survive.

While in Egypt, Antony had been engaged in an affair with Cleopatra and had fathered two children with her. Mark Antony's children with Cleopatra were the twins Alexander Helios and Cleopatra Selene II born in 40 BC, and their son Ptolemy Philadelphus born in 36 BC. Antony's Gallic provinces also fell into Octavian's hands after the death of Antony's legate Quintus Fufius Calenus in 40 BC. Aware of his deteriorating relationship with Octavian, Antony left Cleopatra; he sailed to Italy in 40 BC with a large force to oppose Octavian, laying siege to Brundisium. This new conflict proved untenable for both Octavian and Antony, however. Their centurions, who had become important figures politically, refused to fight because of their Caesarian cause, while the legions under their command followed suit. Meanwhile, in Sicyon, Antony's wife Fulvia died of a sudden illness, shortly after Antony met with her. Fulvia's death and the mutiny of their centurions allowed the two remaining triumvirs to effect a reconciliation.

In the autumn of 40, Octavian and Antony approved the Treaty of Brundisium, by which Lepidus would remain in Africa, Antony in the East, Octavian in the West. The Italian Peninsula was left open to all for the recruitment of soldiers, but in reality this provision was useless for Antony in the East. Octavian was in a stronger negotiating position due to the troubles Antony had to face in the east with the Parthians invading Asia Minor. To further cement relations of alliance with Antony, Octavian gave his sister, Octavia Minor, in marriage to Antony in late 40 BC.

War with Sextus Pompeius and exile of Lepidus

Sextus Pompeius threatened Octavian in Italy by denying shipments of grain through the Mediterranean Sea to the peninsula. Pompeius's own son was put in charge as naval commander in the effort to cause widespread famine in Italy. Pompeius's control over the sea prompted him to take on the name Neptuni filius ('son of Neptune'). Before the battles of Philippi, Octavian had sent Salvidienus Rufus with a naval force to dislodge Sextus Pompeius from Sicily, but without success, and so the triumvirs recognized his command over the sea with the Treaty of Brundisium in 40 BC. However, when Sextus Pompeius resumed his blockade, a starving angry mob in Rome blamed Octavian and Antony and attacked them in early 39 BC; Antony's forces rescued Octavian and dispersed the mob. Another temporary peace agreement was reached in 39 BC with the Pact of Misenum. The blockade on Italy was lifted once Octavian granted Pompeius Sardinia, Corsica, Sicily, and the Peloponnese, and ensured him a future position in the consulship. Sextus Pompeius was also allowed to join the college of augurs.

The territorial agreement between the triumvirate and Sextus Pompeius began to crumble once Octavian divorced Scribonia and married Livia on 17 January 38 BC. After Antony refused to relinquish the Peloponnese, Pompeius blocked Rome's food supply, causing riots there. Pompeius's naval commander Menas betrayed him, handing over three legions to Octavian, as well as Corsica and Sardinia. However, Octavian's naval forces were defeated at Cumae. Octavian lacked the resources to confront Pompeius alone, so an agreement was reached with the triumvirate's extension for another five-year period beginning in 37 BC.

In supporting Octavian, Antony expected to gain support for his own campaign against the Parthian Empire, desiring to avenge Rome's defeat at Carrhae in 53 BC. In an agreement reached at Tarentum in mid-37 BC, Antony provided 120 ships for Octavian to use against Pompeius, while Octavian was to send 20,000 legionaries to Antony for use against Parthia. Two years later Octavian sent only a tenth of those promised, which Antony viewed as an intentional provocation. Meanwhile, Agrippa was tasked with creating the artificial harbor Portus Julius near Cumae by joining the Lucrinus and Avernus lakes for the training and shipbuilding of Octavian's naval fleet.

Octavian and Lepidus launched a joint operation against Sextus in Sicily in 36 BC. Octavian suffered major setbacks, including the loss of his fleet as he became shipwrecked in Sicily, aided by the sole companion Marcus Valerius Messalla Corvinus. However, the naval fleet of Sextus Pompeius fled the fighting against Agrippa at Mylae in August, and was almost entirely destroyed on 3 September by Agrippa at the naval battle of Naulochus. Sextus fled to the east with his remaining forces, where he was captured and executed in Miletus by Antony or one of his legates in 35 BC.

As Lepidus and Octavian accepted the surrender of Pompeius's troops, Lepidus attempted to claim Sicily for himself, ordering Octavian to leave. Lepidus's troops deserted him, however, and defected to Octavian since they were weary of fighting, enticed by Octavian's promises of money. They were also effectively surrounded by Octavian's forces in Sicily. Lepidus surrendered to Octavian and was permitted to retain the office of pontifex maximus, but was ejected from the Triumvirate and exiled to a villa at Cape Circei in Italy. The Roman dominions were then divided between Octavian in the West and Antony in the East. Octavian ensured Rome's citizens of their rights to property, settled his discharged soldiers outside of Italy, and returned 30,000 slaves to their former Roman owners after they had fled to join Pompeius's army and navy. Octavian had the Senate grant him, his wife Livia, and his sister Octavia tribunician immunity, or sacrosanctitas, in order to ensure the safety of the three once he returned to Rome.

After defeating Pompeius, Octavian's military campaigns in Illyricum from 35 to 33 BC secured the submission of the Iapodes and Dalmatae people (in what is now Croatia). This conflict allowed him to return the military standards previously lost by Aulus Gabinius, which Octavian housed afterwards in the Porticus Octavia. During the first campaign, Octavian destroyed Segesta (modern Siscia) and was wounded by a collapsing siege ramp when he besieged Metulum (along the Kolpa River). These efforts were lauded in the Senate, though Octavian postponed a triumph for his victories, and only later acknowledged the contributions of commanders Agrippa and Statilius Taurus. Nevertheless, both Octavian and Antony's generals celebrated triumphs during the 30s BC. For instance, Statilius Taurus celebrated a triumph in 34 BC and later built Rome's first fully stone amphitheater. Antony's general Publius Ventidius when granted a triumph for defending Roman Syria in 38 BC against the invasion of Pacorus I of Parthia.

War with Antony and Cleopatra

In 36 BC, Octavian used a political ploy to make himself look less autocratic and Antony more the villain by proclaiming that the civil wars were coming to an end and that he would step down as triumvir—if only Antony would do the same. Antony refused. Antony's Parthian campaign in 36 BC turned into a debacle, tarnishing his image as a leader. The mere 2,000 legionaries sent by Octavian to Antony, traveling with his wife Octavia, were hardly enough to replenish his lost forces. On the other hand, Cleopatra, with her enormous wealth, could restore his army to full strength. Antony was already engaged in a romantic affair with Cleopatra, their third child Ptolemy Philadelphus born in 36 BC, so in 35 BC he decided to send Octavia back to Rome. Octavian used this to spread propaganda implying that Antony was becoming less than Roman, rejecting his legitimate Roman spouse who he wronged while favoring a foreign queen as his lover.

Roman troops captured the Kingdom of Armenia in 34 BC, and Antony made his son Alexander Helios the ruler of Armenia. Cleopatra assumed the title "Queen of Kings" and her son Caesarion was named King of Kings and co-regent, acts that Octavian used to convince the Senate that Antony had ambitions to diminish the preeminence of Rome. Octavian became consul once again on 1 January 33 BC, and he opened the following session in the Senate with a vehement attack on Antony's grants of titles and territories to his relatives and to his queen, later known as the Donations of Alexandria.

In early 32 BC, amid an intense war of propaganda between him and Octavian, Antony publicly announced the end of his marriage to Octavia. The new consuls Gaius Sosius and Gnaeus Domitius Ahenobarbus were Antonine loyalists who threatened to revoke Octavian's triumviral authority. Southern writes that after consul Gaius Sosius made a speech on 1 January 32 BC denouncing Octavian, a measure brought forth by Sosius was vetoed by the tribune Nonius Balbus. It is unclear what this vetoed legislation contained precisely, but Southern suggests it was likely an attempt to strip Octavian of all his triumviral powers, which had now expired. This prompted Octavian to enter the Senate house with armed guards where he made a speech accusing Antony and Sosius of misdeeds, an intimidation tactic that convinced a large portion of the senators and both consuls to flee Rome and defect to Antony.

However, Octavian received two key deserters from Antony in the autumn of 32 BC: Munatius Plancus and Marcus Titius. These defectors gave Octavian the information that he needed to confirm with the Senate all the accusations that he made against Antony. Octavian forcibly entered the temple of the Vestal Virgins and seized Antony's secret will, which he promptly publicized. The will would have given away Roman-conquered territories as kingdoms for his sons to rule and designated Alexandria as the site of a tomb for him and his queen. Octavian demonstrated his loyalty to Rome by building his mausoleum on the Campus Martius. He also contested the clause in Antony's will recognizing Caesarion as the true heir to Julius Caesar. Southern clarifies about the general Roman view regarding Mark Antony's will in the Donations of Alexandria and Octavian's personal views, writing that "the most ignominious clause, from a Roman point of view," was the desire of Antony to be buried next to Cleopatra in Alexandria. However, according to Southern, "the most damaging clause, from Octavian's more personal point of view," was the fact that Antony had named the children he sired with Cleopatra as his heirs, and recognized Caesarion as the true heir to Julius Caesar, not Octavian. Southern asserts that "this sealed Caesarion's death warrant, as far as Octavian was concerned".

In late 32 BC, the Senate officially revoked Antony's assigned consulship for the following year and declared war on Cleopatra's regime in Egypt. War was declared specifically against Cleopatra and her Ptolemaic Kingdom, not against fellow citizen Mark Antony, which was easier to sell to the Roman people who were wary of further civil wars among Romans, but who could stomach a war against a foreign queen who posed a legitimate threat. Historian Duane W. Roller highlights how the legal grounds for the war were based on the fact that Cleopatra was illicitly arming and supplying troops to a private Roman citizen, Antony, whose triumviral authority had by now technically expired. Following the terms of two suffect consuls for late 32 BC, Octavian had legal standing to conduct the war against Antony by being elected as consul for 31 BC. He retained his authority as a triumvir despite his term having officially expired at the end of 33 BC. He used emergency powers (tumultus) to have men of military age throughout the Republic swear an oath of loyalty to him as a mandate for assuming leadership.

In early 31 BC, Antony and Cleopatra were temporarily stationed in Greece when Octavian gained a preliminary victory: the navy successfully ferried troops across the Adriatic Sea under the command of Agrippa. Agrippa cut off Antony and Cleopatra's main force from their supply routes in the Ionian Sea, while Octavian landed at Toryne in Epirus, opposite the Greek island of Corcyra (modern Corfu), and marched south. Trapped on land and sea, deserters of Antony's army fled to Octavian's side daily while Octavian's forces were comfortable enough to make preparations.

Antony's fleet sailed through the bay of Actium along the Ambracian Gulf of western Greece in a desperate attempt to break free of the naval blockade. It was there that Antony's fleet led by Gaius Sosius faced Octavian's fleet under Agrippa in the Battle of Actium on 2 September 31 BC. Cleopatra and her portion of the fleet withdrew early in the battle and were later joined by Antony; his remaining forces were spared in a last-ditch effort by Cleopatra's fleet that had been waiting nearby. All of Antony's nearby forces on land surrendered to Octavian after initially attempting a retreat through Macedonia. After Actium, Octavian returned to Italy in order to settle affairs there with the legions that were formerly under Mark Antony, ensuring that they were not restless or rebellious by sending them on other campaigns or decommissioning them with land pensions and retirement. Within only 30 days after landing at Brundisium, Octavian set out again for the east, traveling first to Greece, then to Syria, and from there marched his forces into Ptolemaic Egypt. Various client rulers siding with Antony now also defected to Octavian, such as Herod the Great of Judea, who met Octavian at Rhodes and would help supply Octavian's forces at Ptolemais in Phoenicia during their march to Egypt. Octavian would later establish a new city—Nicopolis ('victory city')—near the site of the battle at Actium, where games were staged every four years in honor of his victory.

A year later, Octavian defeated their forces in Alexandria on 1 August 30 BC—after which Antony and Cleopatra committed suicide. Antony fell on his own sword and was allegedly taken by his soldiers back to Cleopatra's tomb where he died in her arms. After meeting with Octavian and refusing to be paraded in a triumph at Rome, Cleopatra took her own life by poisoning, contrary to the popular belief that she was bitten by an asp. Octavian had exploited his position as Caesar's heir to further his own political career, and he was well aware of the dangers in allowing another person to do the same. He followed the advice of the Greek philosopher Arius Didymus that there was room in the world for only one Caesar, and therefore ordered Caesarion to be killed. He also had Antony's son Marcus Antonius Antyllus killed, but spared Antony's younger son Iullus and his children by Cleopatra. Octavian had previously shown little mercy to surrendered enemies and acted in ways that had proven unpopular with the Roman people, yet he pardoned many of his opponents after the Battle of Actium. He also ensured that Cleopatra was buried properly next to Antony in their tomb. After Octavia arranged the marriage of Cleopatra's daughter Cleopatra Selene II to Juba II of Numidia, Emperor Augustus appointed this couple as the new co-rulers of Mauretania in 25 BC.

The conquest of Ptolemaic Egypt relieved all of Octavian's personal financial debts incurred during the civil wars. Octavian controlled Roman Egypt directly, forbade Roman senators to travel there, and appointed equestrian governor Cornelius Gallus to supervise its administration and immensely lucrative taxation. While staying in Alexandria in 30 BC, Octavian visited the tomb of Alexander the Great, the Macedonian king and conqueror he emulated and associated himself with through similarly styled artistic portraits. According to one ancient source, Octavian accidentally knocked off the nose of the body of Alexander in his tomb. Octavian's conquest of Ptolemaic Egypt brought an end to the Hellenistic period initiated by Alexander. It also led to the cultural formation of a Greek East and Latin West across the Mediterranean world and the type of cosmopolitan universal monarchy espoused by Alexander, though one that was now centered on Rome. Octavian would become not only the first Roman emperor as Augustus but also the first Roman pharaoh of Egypt. However, he did not partake in Egyptian coronation rites or worship of the Apis bull, and he never traveled to Egypt again after 30 BC.