Caesar's civil war (49–45 BC) occurred during the late Roman Republic between two factions led by Julius Caesar and Pompey. The main cause of the war was political tensions relating to Caesar's place in the Republic on his expected return to Rome on the expiration of his governorship in Gaul.
Before the war, Caesar had led an invasion of Gaul for almost ten years. A build-up of tensions starting in late 50 BC, with both Caesar and Pompey refusing to back down, led to the outbreak of civil war. Pompey and his allies induced the Senate to demand Caesar give up his provinces and armies in the opening days of 49 BC. Caesar refused and instead marched on Rome.
The war was fought in Italy, Illyria, Greece, Egypt, Africa, and Hispania. The decisive events occurred in 48 BC: Pompey defeated Caesar at the Battle of Dyrrhachium in modern Albania, but the subsequent larger Battle of Pharsalus in Greece was won by Caesar, and Pompey's army disintegrated. Many prominent supporters of Pompey (termed Pompeians) surrendered after the battle, such as Marcus Junius Brutus and Cicero. Others fought on, including Cato the Younger and Metellus Scipio. Pompey fled to Egypt, where he was assassinated upon arrival.
Caesar led a military expedition to Asia Minor before attacking North Africa, where he defeated Metellus Scipio in 46 BC at the Battle of Thapsus. Cato the Younger and Metellus Scipio committed suicide shortly after Caesar's victory. In the following year, Caesar defeated the last of the Pompeians in the Battle of Munda in Spain, who were led by his former lieutenant Titus Labienus. Caesar was then made dictator perpetuo ("dictator in perpetuity" or "dictator for life") by the Senate in 44 BC. He was assassinated by a group of senators (including Brutus) shortly thereafter.
The civil war is one of the commonly recognised endpoints of Rome's republican government. Some scholars view the war as the proximate cause of the republic's fall, due to its polarising interruption of normal republican government. Caesar's comprehensive victory followed by his immediate death left a power vacuum; over the following years his heir Octavian was eventually able to take complete control, forming the Roman Empire as Augustus.
Background
The main issue in the lead-up to the war was how Caesar, who had been in Gaul for almost ten years before 49 BC, was to be re-integrated into the political fabric of Rome after accumulating immense power and wealth in Gaul.
Starting from 58 BC, the year after his consulship in 59, Caesar had held the proconsulship of Cisalpine Gaul along with Illyricum under the terms of the lex Vatinia and Transalpine Gaul at the assignment of the Senate. Caesar had allied himself with Crassus and Pompey in the so-called First Triumvirate during his consulship. The alliance of three men "induced a sharp restructuring of alliances and alignments" with temporary benefit to them but harm in the long-run with aristocratic groups coalescing in opposition. The short-term benefits to the three emerged from their own purposes: ratification of Pompey's eastern settlement and agrarian measures involving Pompey and Crassus.
The political alliance began to fray in the mid 50s BC, but was put on hold with a renegotiation and the joint consulship of Pompey and Crassus in 55 BC. Their joint consulship assigned new provincial commands to the consuls, with Pompey receiving Hispania while Crassus went to Syria to fight the Parthians; Caesar, for his part, had his proconsulship in Gaul renewed.
After Crassus' departure from Rome at the end of 55 BC and following his death in battle in 53 BC, the alliance started to fracture more cleanly. With the death of Crassus and that of Julia (Caesar's daughter and Pompey's wife) in 54 BC, the balance of power between Pompey and Caesar collapsed and "a faceoff between [the two] may, therefore, have seemed inevitable". From 61 BC, the main political fault line in Rome was counterbalancing against the influence of Pompey, leading to his seeking allies outside the core senatorial aristocracy, i.e. Crassus and Caesar; but the rise of anarchic political violence from 55 to 52 BC finally forced the Senate to ally with Pompey to restore order. The breakdown of order in 53 and 52 BC was extremely disturbing; men like Publius Clodius Pulcher and Titus Annius Milo were "essentially independent agents" leading large violent street gangs in a highly volatile political environment. This led to Pompey's sole consulship in 52 BC in which he took sole control of the city without convening an electoral assembly.
Political agitation to strip Caesar of his command and his legions had already started in the spring of 51 BC: M Claudius Marcellus argued in that year that the capture of Alesia and victory over Vercingetorix meant that Caesar's provincia (i.e., task) in Gaul was completed and therefore his command had lapsed. He also argued that Caesar's expected desire to stand for a second consulship in absentia was no longer justified after his victory. Regardless, the Senate rejected Marcellus' motion, as well as his later motion to declare Caesar's term in Gaul to end on 1 March 50 BC. Pompey was also instrumental in rejecting the proposed motions.
After the summer of 50, "positions had been hardened and events progressed irreversibly toward cataclysm", with Pompey now rejecting Caesar's standing for a second consulship until he gave up his army and provinces. The Senate as a whole was relatively pacific, strongly supporting a proposal by Caesar's ally C Scribonius Curio, who was then tribune of the plebs, that both Pompey and Caesar give up their armies and commands. The proposal passed in the Senate by 370 to 22 on 1 December 50 BC, but it was rejected by Pompey and the consul. The consul, C Claudius Marcellus, then seized upon rumours that Caesar was preparing to invade Italy and charged Pompey with defending the city and the Republic.
One of the reasons given as to why Caesar decided to go to war was that he would be prosecuted for legal irregularities during his consulship in 59 BC and violations of various laws passed by Pompey in the late 50s, the consequence of which would be ignominious exile. However, the prosecution theory emerging from Suetonious and Pollio is in "highly dubious territory" and "dubious in the extreme". There is no evidence from the period 50–49 BC that anyone was seriously planning on putting Caesar on trial. Caesar's choice to fight the civil war was motivated by his mostly stumbling in efforts to attain a second consulship and triumph, in which failure to do so would have jeopardised his political future. Moreover, war in 49 BC was advantageous for Caesar, who had continued military preparations while Pompey and the republicans had barely started preparing.
Even in ancient times, the causes of the war were puzzling and perplexing, with specific motives "nowhere to be found". Various pretexts existed, such as Caesar's claim that he was defending the rights of tribunes after they fled the city, which was "too obvious a sham". Caesar's own explanation was that he would protect his personal dignitas; both Caesar and Pompey were impelled by pride, with Caesar refusing to "yield submissively to the blusterings of the conservatives, much less to the bullying of Pompey" in Gruen's words, and Pompey similarly refusing to accept Caesar's proposals, delivered as if they were directives. There was little conscious desire for war until the last weeks of 50 BC, but "the boni had entrapped themselves... in a political vise from which they could not emerge with dignity except by aggressive self-assertion" while Caesar could not "permit [his status and reputation] to collapse through submission".
Civil war
For the months leading up to January 49 BC, both Caesar and the anti-Caesarians (composed of Pompey, Cato, and others) seemed to believe that the other would back down or, failing that, offer acceptable terms. Trust had eroded between the two over the last few years and repeated cycles of brinksmanship harmed chances for compromise.
On 1 January 49 BC, Caesar stated that he would be willing to resign if other commanders would also do so but, in Gruen's words, "would not endure any disparity in their [Caesar and Pompey's] forces", appearing to threaten war if his terms were not met. Caesar's representatives in the city met with senatorial leaders with a more conciliatory message, with Caesar willing to give up Transalpine Gaul if he would be permitted to keep two legions and the right to stand for consul without giving up his imperium (and, thus, right to triumph), but these terms were rejected by Cato, who declared he would not agree to anything unless it was presented publicly before the Senate.
The Senate was persuaded on the eve of war (7 January 49 BC) – while Pompey and Caesar continued to muster troops – to demand Caesar give up his post or be judged an enemy of the state. A few days later, the Senate then also stripped Caesar of his permission to stand for election in absentia and appointed a successor to Caesar's proconsulship in Gaul; while pro-Caesarian tribunes vetoed these proposals, the Senate ignored it and moved the senatus consultum ultimum, empowering the magistrates to take whatever actions were necessary to ensure the safety of the state. In response, a number of those pro-Caesarian tribunes, dramatising their plight, fled the city for Caesar's camp.
Opening of the war
Crossing the Rubicon
On the 10th or 11 January, Caesar crossed the Rubicon, a small river marking the boundary between the province of Cisalpine Gaul to the north and Italy proper to the south. Crossing the Rubicon, Suetonius claims Caesar exclaimed alea iacta est ("the die is cast"), though Plutarch maintains Caesar spoke in Greek quoting the poet Menander with anerriphtho kubos ("ἀνερρίφθω κύβος", "let the die be thrown"); Caesar's own commentaries do not mention the Rubicon at all. This marked a formal start to hostilities, with Caesar being "undoubtedly a rebel".
On both sides, the rank and file soldiers followed their leaders: "the Gallic legions obeyed their patron and benefactor [who] deserved well of the res publica... others followed Pompey and the consuls [who] represented the res publica". Caesar made sure to address his men: according to his own account, he spoke of injustices done to him by his political enemies, how Pompey had betrayed him, and focused mostly on how the rights of tribunes had been trampled by the Senate's ignoring tribunician vetoes, parading the tribunes who had fled the city before the troops in their disguises. On the senatus consultum ultimum, Caesar argued it was unnecessary and should be confined only to circumstances in which Rome was under direct threat.
For most Romans, the choice of which side to pick was difficult. Only a small number of people were committed to one side or the other at the onset of hostilities. For example, Gaius Claudius Marcellus, who as consul in 50 BC had charged Pompey with defending the city, chose neutrality. The then-young Marcus Junius Brutus, whose father had been treacherously killed by Pompey during Brutus' childhood, whose mother was Caesar's lover, and who had been raised in Cato the Younger's house, chose to leave the city, setting off a post in Cilicia and thence to Pompey's camp. Caesar's most trusted lieutenant in Gaul, Titus Labienus, also defected from Caesar to Pompey, possibly due to Caesar's hoarding of military glories or an earlier loyalty to Pompey.
March on Rome
Caesar's timing was far-sighted: Italy was totally unprepared for an invasion. Caesar captured Ariminum (modern day Rimini) without resistance, his men having already infiltrated the city; he captured three more cities in quick succession. News of Caesar's incursion into Italy reached Rome around 17 January. In response Pompey "issued an edict in which he recognised a state of civil war, ordered all the senators to follow him, [and] declared that he would regard as a partisan of Caesar any one who remained behind". Pompey and his allies left the city along with many uncommitted senators, fearing bloody reprisals of the previous civil wars; other senators simply left Rome for their country villas, hoping to keep a low profile.
In late January, Caesar and Pompey were negotiating, with Caesar proposing that the two of them return to their provinces (which would have required Pompey to travel to Spain) and then disband their forces. Pompey accepted those terms provided that they withdraw from Italy at once and submit to arbitration of the dispute by the Senate, a counter-offer that Caesar rejected as doing so would have put him at the mercy of hostile senators while giving up all the advantages of his surprise invasion. Caesar continued to advance.
After encountering five cohorts under Quintus Minucius Thermus at Iguvium, Thermus' forces deserted. Caesar quickly overran Picenum, the area from which Pompey's family originated. While Caesar's troops skirmished once with local forces, fortunately for him, the population was not hostile: his troops were refraining from looting and his opponents had "little popular appeal". In February 49 BC, Caesar received reinforcements and captured Asculum when the local garrison deserted.
Only when he reached Corfinium did he encounter serious opposition led by Lucius Domitius Ahenobarbus, recently appointed governor of Gaul by the Senate. Pompey had urged Ahenobarbus to retreat south and join him, but Ahenobarbus had responded with requests for support; regardless, Caesar prepared for a siege. After Ahenobarbus received a letter from Pompey denying support, he claimed help was on the way but was caught planning a personal escape; in response, his men arrested him and sent envoys to surrender to Caesar after a short week-long siege. Among the surrendered were some fifty senators and equestrians, all of whom Caesar allowed to go free. When Corfinium's local magistrates handed over some six million sestertii that Ahenobarbus had brought to pay his men, Caesar returned it to the men and asked them to take an oath of loyalty, which they did.