The Second Triumvirate (43–32 BC) was a political alliance formed after the Roman dictator Julius Caesar's assassination, comprising Caesar's adopted son Octavian (the future emperor Augustus) and the dictator's two most important supporters, Mark Antony and Marcus Aemilius Lepidus. The triumvirate for organizing the republic (Latin: tresviri rei publicae constituendae), as it was formally known, ruled the Roman Republic essentially as a military dictatorship, with each of the triumvirs assuming charge of an individual set of provinces. Unlike the earlier First Triumvirate (Caesar, Pompey and Crassus), the Second was an official, legally established institution, whose overwhelming power in the Roman state was given full legal sanction and whose authority outranked that of all other magistrates, including the consuls.
The triumvirate was formally recognized by the Senate in the Lex Titia in November of 43 BC, granting the trio supreme authority for five years (until 1 January 37 BC), and assigning them the important task of hunting down the conspirators involved in Caesar's assassination, especially Brutus and Cassius.  It formally deified Caesar as Divus Iulius in 42 BC, and Caesar Octavian henceforth became Divi filius ("Son of the Divine"). To cement the alliance, Antony gave his stepdaughter Claudia to Octavian in marriage and Rome's territories were divided between the triumvirs. Seeing that Caesar's clemency had resulted in his murder, the Second Triumvirate brought back proscription, abandoned since Sulla. It engaged in the legally sanctioned murder of a large number of its opponents in order to fund its forty-five legions in the second civil war against Brutus and Cassius. Antony and Octavian defeated them at Philippi. Around this time, Antony married Octavian's sister, Octavia.
The Second Triumvirate was ultimately unstable and could not withstand internal jealousies and ambitions. Antony detested Octavian and spent most of his time in the East, while Lepidus favoured Antony but felt himself obscured by both his colleagues. Following the war against Sextus Pompeius, a dispute between Lepidus and Octavian regarding the allocation of lands broke out. Octavian accused Lepidus of usurping power in Sicily and of attempted rebellion and, in 36 BC, Lepidus was forced into exile in Circeii and stripped of all his offices except that of Pontifex Maximus. His former provinces were awarded to Octavian. Antony, meanwhile, married Caesar's lover, Cleopatra, intending to use the fabulously wealthy Egypt as a base to dominate Rome. A third civil war subsequently broke out between Octavian on one hand and Antony and Cleopatra on the other. This final civil war culminated in the latter's defeat at Actium in 31 BC; Octavian's forces would then chase Antony and Cleopatra to Alexandria, where they would both commit suicide in 30 BC. With the complete defeat of Antony and the marginalisation of Lepidus, Octavian, having been restyled "Augustus", a name that raised him to the status of a deity, in 27 BC, remained as the sole master of the Roman world and proceeded to establish the Principate as the first Roman "Emperor".
Octavian, despite his youth (20 years old), extorted from the Senate the post of suffect consul (consul suffectus) for the year 43 BC. He had been warring with Antony and Lepidus in upper Italia, but in October 43 BC the three agreed to unite and seize power and so met near Bononia (now Bologna).
This triumvirate of new leaders was established in 43 BC as the Triumviri Rei Publicae Constituendae Consulari Potestate (Triumvirs for Confirming the Republic with Consular Power, abbreviated as III VIR RPC). Where the first triumvirate was essentially a private agreement, the second was embedded in the constitution formally joining Octavian, Antony, and Lepidus in shared rule over Rome. The only other office which had ever been qualified "for confirming the Republic" was the dictatorship of Lucius Cornelius Sulla; the only limit on the powers of the Triumvirate was the five-year term set by law.
A historical oddity of the Triumvirate is that it was, in effect, a three-man directorate with dictatorial powers; it included Antony, who as consul in 44 BC had obtained a lex Antonia that abolished the dictatorship and expunged it from the Republic's constitutions. As had been the case with both Sulla and Julius Caesar during their dictatorships, the members of the Triumvirate saw no contradiction between holding a supraconsular office and the consulate itself simultaneously.
In 44 BC, Lepidus' possession of the provinces of Hispania and Narbonese Gaul was confirmed, and he agreed to hand over 7 legions to Octavian and Antony to continue the struggle against Brutus and Cassius for eastern Roman territory; in the event of defeat, Lepidus' territories would provide a fall-back position. Antony retained Cisalpine Gaul and hegemony over Gaul itself, and Octavian held Africa and was given nominal authority over Sicily and Sardinia. According to historian Richard Weigel, Octavian's share at this stage was "practically humiliating"; all the most important provinces went to Antony and Lepidus, though transfer of Lepidus' legions to Octavian meant that Lepidus was "effectively eliminating himself as an equal partner" in future.)
In order to refill the treasury, the Triumvirs decided to resort to proscription. As all three had been partisans of Caesar, their main targets were opponents of the Caesarian faction. The most notable victims were Marcus Tullius Cicero, who had opposed Caesar and excoriated Antony in his Philippicae, and Marcus Favonius, a follower of Cato and an opponent of both triumvirates. The proscription of Caesar's legate Quintus Tullius Cicero (Marcus Tullius Cicero's younger brother) seems to have been motivated by the perceived need to destroy Cicero's family. For ancient writers, the most shocking proscriptions were those of Caesar's own cousin Lucius Julius Caesar, and Lepidus' brother Lucius Aemilius Lepidus Paullus. They were added to the list because they had been the first to condemn Antony and Lepidus after the two allied. In fact they both survived.
Octavian's colleague in the consulate that year, his cousin (and nephew of Caesar), Quintus Pedius, died before the proscriptions got underway. Octavian himself resigned shortly after, allowing the appointment of a second pair of suffect consuls; the original consuls for the year, Caesar's legate Aulus Hirtius and Gaius Vibius Pansa Caetronianus, had died fighting on the Senate's side of the first civil war to follow Caesar's death, that between the Senate and Mark Antony himself. This became a broad pattern of the Triumvirate's two terms; during the ten years of the Triumvirate (43 BC to 33 BC), there were 42 consuls in office, rather than the expected 20.
The Caesarian background of the Triumvirs made it no surprise that immediately after the conclusion of the first civil war of the post-Caesar period, they immediately set about prosecuting a second: Caesar's murderers Marcus Junius Brutus and Gaius Cassius Longinus had usurped control of most of the Eastern provinces, including Macedonia, Asia Minor, and Syria. In 42 BC, Octavian and Antony set out to war, defeating Brutus and Cassius in two battles fought at Philippi.
After the victory, Antony and Octavian agreed to divide the provinces of the Republic into spheres of influence. Octavian—who had begun calling himself "Divi filius" ("son of the divinity") after Caesar's deification as Divus Julius ("the Divine Julius") and now styled himself simply "Imperator Caesar"—took control of the West, Antony of the East. As a result, the province of Cisalpine Gaul was absorbed into Italy. Narbonese Gaul was absorbed into Gallia Comata, creating a unified Gaul, and was thus taken over by Antony. Octavian took over Spain from Lepidus. Lepidus himself was left with nothing, but was offered the prospect of control over Africa. The excuse given for this was a report that Lepidus had been traitorously negotiating with Sextus Pompey. If he were proved innocent he would have Africa. Octavian returned to Rome to administer the distribution of land to his veterans. Antony remained in the east to bring Brutus and Cassius' former territories under triumvirate control.
The reduced role of Lepidus is evident in the fact that far fewer coins depict him from this point on, and a number of triumviral edicts are issued in the names of Antony and Octavian only.
Octavian's land redistribution caused widespread tensions, as farmers were dispossessed in favor of soldiers. Antony's brother Lucius Antonius, who was serving as Consul, stood up for the dispossessed farmers. The conflict led to the Perusine War, in which Lucius gathered an army of supporters to challenge Octavian. He was encouraged by Mark Antony's wife Fulvia. Lepidus held Rome with two legions while Octavian left to gather his army, but Lucius defeated Lepidus, who was forced to flee to Octavian. As Octavian advanced on Rome, Lucius withdrew to Perusia (Perugia), where he was besieged by Octavian in the winter of 41–40 BC. He finally surrendered in exchange for clemency. The outcome was that Lepidus was confirmed as governor of Africa, acquiring six of Antony's legions, leaving Octavian as the sole power in Italy, with his own loyal legions in control. When Antony's supporter Calenus, governor of Gaul, died, Octavian took over his legions, further strengthening his control over the west. This new distribution of power among the triumvirs was confirmed by the Treaty of Brundisium in September 40 BC. At around the same time, Antony's wife Fulvia died. Octavian arranged for Antony to marry his sister, Octavia, as a symbol of the renewed alliance.
The economic problems caused by the eviction of established farmers were exacerbated by the control of Sextus Pompey over Sicily, Corsica and Sardinia. Pompey's navy regularly intercepted Roman shipping, leading to problems with the grain supply. In 39 BC Antony and Octavian decided to negotiate an agreement to stop the piracy. According to Appian, Sextus hoped to replace Lepidus as the third triumvir, but instead he was confirmed in possession of the islands by the Pact of Misenum, in return agreeing to stop his piracy. According to one source Sextus' second-in-command Menas advised him to kidnap and kill Antony and Octavian while they were celebrating the deal at a dinner on Sextus's flagship, but Sextus refused.
Despite the agreement, conflicts continued. Octavian accused Sextus of continuing to raid Italian towns. In the following year Octavian attempted to take Sicily by force. He was defeated twice in naval battles off Messina. He then arranged a meeting with Antony, who was planning to attack Parthia and needed troops. Antony agreed to deliver ships for the attack on Sextus in exchange for troops to fight the Parthians. Octavian also secured the support of Lepidus, planning a simultaneous joint attack on Sicily.
Though Octavian nominally oversaw the campaign against Sextus, the campaign was actually commanded by Octavian's lieutenant, Marcus Vipsanius Agrippa, which culminated in victory in 36 BC. Agrippa had been consul in 37 BC and had secured the Triumvirate's renewal for a second five-year term.
Like the First Triumvirate, the Second Triumvirate was ultimately unstable and could not withstand internal jealousies and ambitions. Antony detested Octavian and spent most of his time in the East, while Lepidus favoured Antony but felt himself obscured by both his colleagues, despite having succeeded Caesar as Pontifex Maximus in 43 BC. During the campaign against Sextus Pompey, Lepidus had raised a large army of 14 legions and a considerable navy. Lepidus had been the first to land troops in Sicily and had captured several of the main towns. However, he felt that Octavian was treating him as a subordinate rather than an equal. This led to an ill-judged political move that gave Octavian the excuse he needed to remove Lepidus from power. After the defeat of Sextus Pompey, Lepidus stationed his legions in Sicily and argued that it should be absorbed into his territories. Alternatively, he should be restored to his former provinces, which had been legally guaranteed by the Lex Titia. Octavian accused Lepidus of attempting to usurp power and fomenting rebellion. Humiliatingly, Lepidus' legions in Sicily defected to Octavian and Lepidus himself was forced to submit to him. Lepidus was stripped of all his offices except that of Pontifex Maximus. Octavian sent him into exile in Circeii.
In order to provide treasures and rewards for his troops and cement his reputation as a military commander, Octavian pursued a war in Illyricum to bring it under Roman control. Meanwhile, Antony was preparing his war against Parthia, taking advantage of divisions caused by the new Parthian king Phraates IV. However Antony over-extended himself and was forced to retreat with considerable loss of troops.
Despite having married Octavia, Octavian's sister, in 40 BC (Octavian had married Antony's stepdaughter Claudia three years earlier), Antony openly lived in Alexandria with Cleopatra VII of Egypt, even siring children with her. When the Triumvirate's second term expired in 33 BC, Antony continued to use the title Triumvir; Octavian, opting to distance himself from Antony, refrained from using it.
After Antony's defeat in Parthia, Cleopatra had come to his aid with supplies; Antony then turned his attention to Armenia, seizing its king Artavasdes and occupying the country. He minted coins to commemorate the victory, created a mimic of a Roman triumph, and read out a declaration, known as the Donations of Alexandria in which he granted territories to Cleopatra's children.
Octavian illegally obtained Antony's will in July 32 BC and exposed it to the Roman public: it promised substantial legacies to Antony's children by Cleopatra, and left instructions for shipping his body to Alexandria for burial. Octavian's forces decisively defeated those of Antony and Cleopatra at the Battle of Actium in Greece in September 31 BC, chasing them to Egypt in 30 BC; both Antony and Cleopatra committed suicide in Alexandria, and Octavian personally took control of Egypt and Alexandria (Egyptian chronologies treat Octavian as Cleopatra's successor as Pharaoh).
Octavian's ally Gaius Maecenas forestalled a conspiracy allegedly organised by Lepidus's son (31 BC). With the complete defeat of Antony and the marginalisation of Lepidus, Octavian, having been restyled "Augustus" in 27 BC, remained as the sole master of the Roman world, and proceeded to establish the Principate as the first Roman "emperor".
After his defeat at Forum Gallorum in 43, Antony fled to join Lepidus at Lugdunum. In the meantime, the Senate refused Octavian his rightful recognition for the victory and commanded him to turn over the troops of the consuls to Brutus. Instead Octavian marched on Rome and forced the Senate to name him consul suffectus.33 Afterwards, he returned to Gaul to complete the campaign against Antony.