The Great Roman Civil War (49–45 BC), also known as Caesar's Civil War, was one of the last politico-military conflicts in the Roman Republic before the establishment of the Roman Empire. It began as a series of political and military confrontations, between Julius Caesar (100–44 BC), his political supporters (broadly known as Populares), and his legions, against the Optimates (or Boni), the politically conservative and socially traditionalist faction of the Roman Senate, who were supported by Pompey (106–48 BC) and his legions.
Prior to the war, Caesar had served for eight years in the Gallic Wars. He and Pompey had, along with Marcus Licinius Crassus, established the First Triumvirate, through which they shared power over Rome. Caesar soon emerged as a champion of the common people, and advocated a variety of reforms. The Senate, fearful of Caesar, demanded that he relinquish command of his army. Caesar refused, and instead marched his army on Rome, which no Roman general was permitted to do. Pompey fled Rome and organized an army in the south of Italy to meet Caesar.
The war was a five-year-long (49–45 BC) politico-military struggle, fought in Italy, Illyria, Greece, Egypt, Africa, and Hispania. Pompey defeated Caesar in 48 BC at the Battle of Dyrrhachium, but was himself defeated much more decisively at the Battle of Pharsalus. The Optimates under Marcus Junius Brutus and Cicero surrendered after the battle, while others, including those under Cato the Younger and Metellus Scipio fought on. Pompey fled to Egypt and was killed upon arrival. Scipio was defeated in 46 BC at the Battle of Thapsus in North Africa. He and Cato committed suicide shortly after the battle. The following year, Caesar defeated the last of the Optimates in the Battle of Munda and became Dictator perpetuo (Dictator in perpetuity) of Rome. The changes to Roman government concomitant to the war mostly eliminated the political traditions of the Roman Republic (509–27 BC) and led to the Roman Empire (27 BC–AD 476).
Caesar's Civil War resulted from the long political subversion of the Roman Government's institutions, begun with the career of Tiberius Gracchus, continuing with the Marian reforms of the legions, the bloody dictatorship of Lucius Cornelius Sulla, and completed by the First Triumvirate over Rome.
The First Triumvirate (so denominated by Cicero), comprising Julius Caesar, Crassus, and Pompey, ascended to power with Caesar's election as consul, in 59 BC. The First Triumvirate was unofficial, a political alliance the substance of which was Pompey's military might, Caesar's political influence, and Crassus' money. The alliance was further consolidated by Pompey's marriage to Julia, daughter of Caesar, in 59 BC. At the conclusion of Caesar's first consulship, the Senate (rather than granting him a provincial governorship) tasked him with watching over the Roman forests. This job, specially created by his Senate enemies, was meant to occupy him without giving him command of armies, or garnering him wealth and fame.
Caesar, with the help of Pompey and Crassus, evaded the Senate's decrees by legislation passed through the popular assemblies. By these acts, Caesar was promoted to Roman Governor of Illyricum and Cisalpine Gaul. Transalpine Gaul (southern France) was added later. The various governorships gave Caesar command of an army of (initially) four legions. The term of his proconsulship, and thus his immunity from prosecution, was set at five years, rather than the customary one year. His term was later extended by another five years. During this ten-year period, Caesar used his military forces to conquer Gaul and invade Britain, without explicit authorisation by the Senate.
In 52 BC, at the First Triumvirate's end, the Roman Senate supported Pompey as sole consul; meanwhile, Caesar had become a military hero and champion of the people. Knowing he hoped to become consul when his governorship expired, the Senate, politically fearful of him, ordered he resign command of his army. In December of 50 BC, Caesar wrote to the Senate agreeing to resign his military command if Pompey followed suit. Offended, the Senate demanded he immediately disband his army, or be declared an enemy of the people: an illegal political bill, for he was entitled to keep his army until his term expired.
A secondary reason for Caesar's immediate desire for another consulship was to delay the inevitable senatorial prosecutions awaiting him upon retirement as governor of Illyricum and Gaul. These potential prosecutions were based upon alleged irregularities that occurred in his consulship and war crimes committed in his Gallic campaigns. Moreover, Caesar loyalists, the tribunes Mark Antony and Quintus Cassius Longinus, vetoed the bill, and were quickly expelled from the Senate. They then joined Caesar, who had assembled his army, whom he asked for military support against the Senate; agreeing, his army called for action.
In 50 BC, at his Proconsular term's expiry, the Pompey-led Senate ordered Caesar's return to Rome and the disbanding of his army, and forbade his standing for election in absentia for a second consulship; because of that, Caesar thought he would be prosecuted and rendered politically marginal if he entered Rome without consular immunity or his army; to wit, Pompey accused him of insubordination and treason.
On 10 January 49 BC, commanding the Legio XIII Gemina, Caesar crossed the Rubicon River, the boundary between the province of Cisalpine Gaul to the north and Italy proper to the south. As crossing the Rubicon with an army was prohibited, lest a returning general attempt a coup d'etat, this triggered the ensuing civil war between Caesar and Pompey.
The general population, who regarded Caesar as a hero, approved of his actions. The historical records differ about which decisive comment Caesar made on crossing the Rubicon: one report is Alea iacta est (usually translated as "The die is cast").
Caesar's march on Rome was a triumphal procession. The Senate, not knowing that Caesar possessed only a single legion, feared the worst and supported Pompey. Pompey declared that Rome could not be defended; he escaped to Capua with those politicians who supported him, the aristocratic Optimates and the regnant consuls. Cicero later characterised Pompey's "outward sign of weakness" as allowing Caesar's consolidation of power.
Despite having retreated into central Italy, Pompey and the Senatorial forces were composed of at least two legions: some 11,500 soldiers and some hastily levied Italian troops commanded by Lucius Domitius Ahenobarbus. As Caesar progressed southwards, Pompey retreated towards Brundisium, initially ordering Domitius (engaged in raising troops in Etruria) to stop Caesar's movement on Rome from the direction of the Adriatic seaboard.
Belatedly, Pompey ordered Domitius to retreat south also, and rendezvous with Pompey's forces. Domitius mostly ignored Pompey's orders, and, after being isolated and trapped near Corfinium, was forced to surrender almost thirty cohorts of troops (about three legions), most of whom promptly joined Caesar's army.
Pompey escaped to Brundisium, there awaiting sea transport for his legions, to Epirus, in the Republic's eastern Greek provinces, expecting his influence to yield money and armies for a maritime blockade of Italy proper. Meanwhile, the aristocrats—including Metellus Scipio and Cato the Younger—joined Pompey there, whilst leaving a rear guard at Capua.
Caesar pursued Pompey to Brundisium, expecting restoration of their alliance of ten years prior; throughout the Great Roman Civil War's early stages, Caesar frequently proposed to Pompey that they, both generals, sheathe their swords. Pompey refused, legalistically arguing that Caesar was his subordinate and thus was obligated to cease campaigning and dismiss his armies before any negotiation. As the Senate's chosen commander, and with the backing of at least one of the current consuls, Pompey commanded legitimacy, whereas Caesar's military crossing of the Rubicon rendered him a de jure enemy of the Senate and People of Rome. Nevertheless, in March 49 BC, Pompey escaped Caesar at Brundisium, fleeing by sea to Epirus.
Taking advantage of Pompey's absence from the Italian mainland, Caesar effected an astonishingly fast 27-day, north-bound forced march to destroy, in the Battle of Ilerda, Hispania's politically leader-less Pompeian army, commanded by the legates Lucius Afranius and Marcus Petreius, afterwards pacifying Roman Hispania; during the campaign, the Caesarian forces—six legions, 3,000 cavalry (Gallic campaign veterans), and Caesar's 900-horse personal bodyguard—suffered only 70 men killed in action, while the Pompeian casualties numbered 200 men killed and 600 wounded.
Returning to Rome in December of 49 BC, Caesar was appointed Dictator, with Mark Antony as his Master of the Horse. Caesar kept his dictatorship for eleven days, tenure sufficient to win him a second term as consul with Publius Servilius Vatia Isauricus as his colleague. Afterwards, Caesar renewed his pursuit of Pompey in Greece.
From Brundisium, Caesar crossed the Strait of Otranto with seven legions to the Gulf of Valona (not Palaesta in Epirus [modern Palase/Dhermi, Albania], as reported by Lucan),  prompting Pompey to consider three courses of action: (i) alliance with the King of Parthia, an erstwhile ally, far to the east; (ii) invade Italy with his superior navy; and/or (iii) force a decisive battle with Caesar. A Parthian alliance was not feasible: a Roman general fighting Roman legions with foreign troops was craven; and the military risk of an Italian invasion was politically unsavoury, because the Italians (who thirty years earlier had rebelled against Rome) might rise against him. Thus, on the advice of his councillors, Pompey decided to engineer a decisive battle.
As it turned out, Pompey would have been obliged to take the third option anyway, as Caesar had forced his hand by pursuing him to Illyria, so, on 10 July 48 BC, the two fought in the Battle of Dyrrhachium. With a loss of 1,000 veteran legionaries Caesar was forced to retreat southwards. Disbelieving that his army had bested Caesar's legions, Pompey misinterpreted the retreat as a feint to a trap, and refused to give chase for the decisive coup de grâce, thus losing the initiative, and the chance to quickly conclude the war. Near Pharsalus, Caesar pitched a strategic bivouac. Pompey attacked, but, despite his much larger army, he was conclusively defeated by Caesar's troops. A major reason for Pompey's defeat was a miscommunication among front cavalry horsemen.
Pompey fled to Egypt, where he was murdered by an officer of King Ptolemy XIII. Caesar pursued the Pompeian army to Alexandria, where he camped and became involved with the Alexandrine civil war between Ptolemy and his sister, wife, and co-regent, Cleopatra VII. Perhaps as a result of Ptolemy's role in Pompey's murder, Caesar sided with Cleopatra; he is reported to have wept at the sight of Pompey's head, which was offered to him by Ptolemy's chamberlain Pothinus as a gift.
In any event, Caesar was besieged at Alexandria and after Mithridates relieved the city, Caesar defeated Ptolemy's army and installed Cleopatra as ruler, with whom he fathered his only known biological son, Ptolemy XV Caesar, better known as "Caesarion". Caesar and Cleopatra never married, due to Roman law that prohibited a marriage with a non-Roman citizen.
After spending the first months of 47 BC in Egypt, Caesar went to Syria, and then to Pontus to deal with Pharnaces II, a client king of Pompey's who had taken advantage of the civil war to attack the Roman-friendly Deiotarus and make himself the ruler of Colchis and lesser Armenia. At Nicopolis Pharnaces had defeated what little Roman opposition the governor of Asia, Gnaeus Domitius Calvinus, could muster. He had also taken the city of Amisus, which was a Roman ally, made all the boys eunuchs, and sold the inhabitants to slave traders. After this show of strength, Pharnaces drew back to pacify his new conquests.
Nevertheless, the extremely rapid approach of Caesar in person forced Pharnaces to turn his attention back to the Romans. At first, recognizing the threat, he made offers of submission, with the sole object of gaining time until Caesar's attention fell elsewhere, to no avail; Caesar quickly routed Pharnaces at the Battle of Zela (modern Zile in Turkey) with just a small detachment of cavalry. Caesar's victory was so swift and complete that, in a letter to a friend in Rome, he famously said of the short war, "Veni, vidi, vici" ("I came, I saw, I conquered"). Indeed, for his Pontic triumph, that may well have been the label displayed above the spoils.
Pharnaces himself fled quickly back to the Bosporus, where he managed to assemble a small force of Scythian and Sarmatian troops, with which he was able to gain control of a few cities; however, a former governor of his, Asandar, attacked his forces and killed him. The historian Appian states that Pharnaces died in battle; Cassius Dio says Pharnaces was captured and then killed.
While Caesar had been in Egypt installing Cleopatra as sole ruler, four of his veteran legions encamped outside of Rome under the command of Mark Antony. The legions were waiting for their discharges and the bonus pay Caesar had promised them before the battle of Pharsalus. As Caesar lingered in Egypt, the situation quickly deteriorated. Antony lost control of the troops and they began looting estates south of the capital. Several delegations of diplomats were dispatched to try to quell the mutiny.
Nothing worked and the mutineers continued to call for their discharges and back pay. After several months, Caesar finally arrived to address the legions in person. Caesar knew he needed these legions to deal with Pompey's supporters in north Africa, who had mustered 14 legions of their own. Caesar also knew that he did not have the funds to give the soldiers their back pay, much less the money needed to induce them to reenlist for the north African campaign.
When Caesar approached the speaker's dais, a hush fell over the mutinous soldiers. Most were embarrassed by their role in the mutiny in Caesar's presence. Caesar asked the troops what they wanted with his cold voice. Ashamed to demand money, the men began to call out for their discharge. Caesar bluntly addressed them as "citizens" instead of "soldiers," a tacit indication that they had already discharged themselves by virtue of their disloyalty.
He went on to tell them that they would all be discharged immediately. He said he would pay them the money he owed them after he won the north African campaign with other legions. The soldiers were shocked. They had been through 15 years of war with Caesar and they had become fiercely loyal to him in the process. It had never occurred to them that Caesar did not need them.
The soldiers' resistance collapsed. They crowded the dais and begged to be taken to north Africa. Caesar feigned indignation and then allowed himself to be won over. When he announced that he would suffer to bring them along, a huge cheer arose from the assembled troops. Through this reverse psychology, Caesar reenlisted four enthusiastic veteran legions to invade north Africa without spending a single sesterce.
Caesar quickly gained a significant victory at Thapsus in 46 BC over the forces of Metellus Scipio, Cato the Younger and Juba (who all committed suicide).
Nevertheless, Pompey's sons Gnaeus Pompeius and Sextus Pompeius, together with Titus Labienus (Caesar's former propraetorian legate (legatus propraetore) and second in command in the Gallic War) escaped to Hispania. Caesar gave chase and defeated the last remnants of opposition in the Battle of Munda in March 45 BC. During this time, Caesar was elected to his third and fourth terms as consul in 46 BC (with Marcus Aemilius Lepidus) and 45 BC (sine collega, without a colleague).
Caesar was later proclaimed dictator first for ten years and then in perpetuity. The latter arrangement triggered the conspiracy leading to his assassination on the Ides of March in 44 BC. Following this, Antony and Caesar's adopted son Octavius would fight yet another civil war against remnants of the Optimates and Liberatores faction, ultimately resulting in the establishment of the Roman Empire.