After Crassus' departure from Rome at the end of 55 BC and following his death in battle in 53 BC, the First Triumvirate 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 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 55–52 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.

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. Caesar's choice to fight the civil war was motivated 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".

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 sar 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.

Caesar had been appointed to a governorship over a region that ranged from southern Gaul to Illyricum. As his term of governorship ended, the Senate ordered Caesar to disband his army and return to Rome.

In January 49 BC C. Julius Caesar led a single legion, Legio XIII, south over the Rubicon from Cisalpine Gaul to Italy to make his way to Rome. In doing so, he deliberately broke the law on imperium and made armed conflict inevitable. Roman historian Suetonius depicts Caesar as undecided as he approached the river and attributes the crossing to a supernatural apparition. It was reported that Caesar dined with Sallust, Hirtius, Oppius, Lucius Balbus and Sulpicus Rufus on the night after his famous crossing into Italy on 10 January.

Caesar's most trusted lieutenant in Gaul, Titus Labienus defected from Caesar to Pompey, possibly due to Caesar's hoarding of military glories or an earlier loyalty to Pompey.

According to Suetonius, Caesar uttered the famous phrase ālea iacta est ("the die has been cast"). The phrase "crossing the Rubicon" has survived to refer to any individual or group committing itself irrevocably to a risky or revolutionary course of action, similar to the modern phrase "passing the point of no return". Caesar's decision for swift action forced Pompey, the consuls and a large part of the Roman Senate to flee Rome. Julius Caesar's crossing of the river precipitated the Great Roman Civil War.

News of Caesar's incursion into Italy reached Rome around 17 January. In response Pompey "issued an edict in which he recognized 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". This led his allies to leave 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.

Caesar's timing was far-sighted: while Pompey's forces actually vastly outnumbered Caesar's single legion, composing at least 100 cohorts, or 10 legions, "by no stretch of the imagination could Italy have been described as prepared to meet 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.

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.

The siege of Corfinium was the first significant military confrontation of Caesar's Civil War. Undertaken in February 49 BC, it saw the forces of Gaius Julius Caesar's Populares besiege the Italian city of Corfinium, which was held by a force of Optimates under the command of Lucius Domitius Ahenobarbus. The siege lasted only a week, after which the defenders surrendered themselves to Caesar. This bloodless victory was a significant propaganda coup for Caesar and hastened the retreat of the main Optimate force from Italia, leaving the Populares in effective control of the entire peninsula.

Caesar's stay at Corfinium lasted seven days in total and after accepting its surrender he immediately broke camp and set out into Apulia to pursue Pompey. Upon learning of Caesar's victory Pompey began to march his army from Luceria to Canusium and then on to Brundisium where he could further retreat by crossing the Adriatic Sea to Epirus. As he began his march Caesar had with him six legions, having immediately sent Ahenobarbus' legions under Curio to secure Sicily; they would later fight for him in Africa. Pompey would be soon be besieged in Brundisium by Caesar's army, though despite this his evacuation was a success.

Caesar's advance down the Adriatic coast was surprisingly clement and disciplined: his soldiers did not plunder the countryside as soldiers had during the Social War a few decades earlier; Caesar did not avenge himself on his political enemies as Sulla and Marius had done. The policy of clemency was also highly practical: Caesar's pacificity prevented the population of Italy from turning on him. At the same time, Pompey planned to escape east to Greece where he could raise a massive army from the eastern provinces. He therefore escaped to Brundisium (modern Brindisi), requisitioning merchant vessels to travel the Adriatic.

Julius Caesar's besieges the Italian city of Brundisium on the coast of the Adriatic Sea which was held by a force of Optimates under the command of Gnaeus Pompeius Magnus. After a series of brief skirmishes, during which Caesar tried to blockade the harbour, Pompey abandoned the city and managed to evacuate his men across the Adriatic to Epirus. Pompey's retreat meant that Caesar had full control over the Italian Peninsula, with no way to pursue Pompey's forces in the east he instead decided to head west to confront the legions Pompey had stationed in Hispania.

On his way to Hispania, Caesar took the opportunity to return to Rome for the first time in nine years. He wished to appear as though he was the legitimate representative of the Republic and so he arranged for the Senate to meet with him outside the boundaries of the city on 1 April. Also invited was the great orator Cicero to whom Caesar sent letters imploring him to come to Rome, but Cicero was not to be persuaded as he was determined not to be used and was wary of the increasingly ominous tone of the letters.

Leaving Mark Antony in charge of Italy, Caesar set out west for Spain. En route, he started a siege of Massilia when the city barred him entry and came under the command of the aforementioned Domitius Ahenobarbus. Leaving a besieging force, Caesar continued to Spain with a small bodyguard and 900 German auxiliary cavalry.

After the siege had begun, Ahenobarbus arrived in Massilia to defend it against the Caesarian forces. In late June, Caesar's ships, although they were less skilfully built than those of the Massiliots and outnumbered, were victorious in the ensuing naval battle.

Gaius Trebonius conducted the siege using a variety of siege machines including siege towers, a siege-ramp, and a "testudo-ram". Gaius Scribonius Curio, careless in adequately guarding the Sicilian Straits, allowed Lucius Nasidius to bring more ships to the aid of Ahenobarbus. He fought a second naval battle with Decimus Brutus in early September, but withdrew defeated and sailed for Hispania.

At the final surrender of Massilia, Caesar showed his usual leniency and Lucius Ahenobarbus fled to Thessaly in the only vessel that was able to escape from the Populares. Afterwards, Massilia was allowed to keep nominal autonomy, due to ancient ties of friendship and support of Rome, along with some territories while most of its empire was confiscated by Julius Caesar.

Caesar arrived in Hispania June 49 BCE, where he was able to seize the Pyrenees passes defended by the Pompeian Lucius Afranius and Marcus Petreius. At Ilerda he defeated a Pompeian army under legates Lucius Afranius and Marcus Petreius. Unlike many of the other battles of the civil war, this was more a campaign of manoeuvre than actual fighting. 

After the surrender of the republican main army in Spain, Caesar then marched towards Varro in Hispania Ulterior, who at once without a fight submitted to him leading to another two legions surrendering.

After this, Caesar left his legate Quintus Cassius Longinus —the brother of Gaius Cassius Longinus— in command of Spain with four of the legions, partly made up of men who had surrendered and gone over to the Caesarian camp, and returned with the rest of his army to Massilia and its siege.

The siege of Curicta was a military confrontation that took place during the early stages of Caesar's Civil War. Occurring in 49 BC, it saw a significant force of Populares commanded by Gaius Antonius besieged on the island of Curicta by an Optimate fleet under Lucius Scribonius Libo and Marcus Octavius. It immediately followed and was the result of a naval defeat by Publius Cornelius Dolabella and Antonius eventually capitulated under prolonged siege. These two defeats were some of the most significant suffered by the Populares during the civil war.

The battle was regarded as a disaster for the Caesarian cause. It seems to have had considerable significance to Caesar who mentions it alongside the death of Curio as one of the worst setbacks of the civil war. Of the four instances that Suetonius gives of the most disastrous defeats suffered by Populares in the civil war, both the defeat of Dolabella's fleet and the capitulation of the legions at Curicta are listed.

The Battle of Tauroento was a naval battle fought off the coast of Tauroento during Caesar's Civil War. Following a successful naval battle outside Massilia, the Caesarian fleet commanded by Decimus Junius Brutus Albinus once again came into conflict with the Massiliot fleet and a Pompeian relief fleet led by Quintus Nasidius on 31 July 49 BC. Despite being significantly outnumbered, the Caesarians prevailed and the Siege of Massilia was able to continue leading to the eventual surrender of the city.

The naval victory at Tauroento meant that the siege of Massilia could continue with a naval blockade in place. Nasidius decided that, given the state of the Massiliot fleet, it would be prudent to lend his support to Pompey's forces in Hispania Citerior rather than continue to assist operations in Gaul. The city of Massilia was dismayed to learn of the destruction of their fleet but nonetheless prepared for many more months under siege. Soon after the defeat Ahenobarbus fled from Massilia and managed to escape capture under cover of a violent storm.

The Battle of Utica (49 BC) in Caesar's Civil War was fought between Julius Caesar's general Gaius Scribonius Curio and Pompeian legionaries commanded by Publius Attius Varus supported by Numidian cavalry and foot soldiers sent by King Juba I of Numidia. Curio defeated the Pompeians and Numidians and drove Varus back into the town of Utica.

In the confusion of the battle, Curio was urged to take the town before Varus could regroup, but he held himself back, as he did not have the means at hand to undertake an assault of the town. The next day however, he began to form a contravallation of Utica, with the intent of starving the town into submission. Varus was approached by the leading citizens of the town, who begged him to surrender and spare the town the horrors of a siege. Varus, however, had just learned that King Juba was on his way with a large force, and so reassured them that with Juba's assistance, Curio would soon be defeated. Curio heard similar reports and abandoned the siege, making his way to the Castra Cornelia. False reports from Utica about Juba's strength caused him to drop his guard, leading to the Battle of the Bagradas River.

After getting the better of Varus’s Numidian allies in a number of skirmishes, he defeated Varus at the Battle of Utica, who fled into the town of Utica. In the confusion of the battle, Curio was urged to take the town before Varus could regroup, but he held himself back, as he did not have the means at hand to undertake an assault of the town. The next day however, he began to form a contravallation of Utica, with the intent of starving the town into submission. Varus was approached by the leading citizens of the town, who begged him to surrender and spare the town the horrors of a siege. Varus, however, had just learned that King Juba was on his way with a large force, and so reassured them that with Juba's assistance, Curio would soon be defeated. Curio, also hearing that Juba’s army was less than 23 miles from Utica, abandoned the siege, making his way to his base on the Castra Cornelia.

Gaius Scribonius Curio was decisively defeated by the Pompeians under Attius Varus and King Juba I of Numidia. One of Curio's legates, Gnaeus Domitius, rode up to Curio with a handful of men, and urged him to flee and make it back to the camp. Curio queried how he could ever look Caesar in the face after he had lost him his army, and turning to face the oncoming Numidians, fought on until he was killed. Only a few soldiers managed to escape the bloodbath that followed, while the three hundred cavalry that had not followed Curio into battle returned to the camp at Castra Cornelia, bearing the bad news.

Returning to Rome in December 49 BC, Caesar left Quintus Cassius Longinus in command of Spain and had praetor Marcus Aemilius Lepidus appoint him dictator. As dictator, he conducted elections for the consulship of 48 BC before using the dictatorial powers to pass laws recalling from exile those condemned by Pompey's courts in 52 BC, excepting Titus Annius Milo, and restoring the political rights of the children of victims of the Sullan proscriptions. Holding the dictatorship would have been the only way to avoid giving up his imperium, legions, provincia, and right to triumph while within the pomerium. Standing in the same elections he conducted, he won a second term as consul with Publius Servilius Vatia Isauricus as his colleague. He resigned the dictatorship after eleven days. Caesar then renewed his pursuit of Pompey across the Adriatic.

On 4 January 48 BC, Caesar moved seven legions – most likely below half-strength – onto a small fleet he assembled and crossed the Adriatic. Caesar's opponent in the consulship of 59 BC, Marcus Calpurnius Bibulus, was in charge of defending the Adriatic for the Pompeians: Caesar's decision to sail, however, surprised Bibulus' fleet. Caesar landed at Paeleste, on the Epirot coast, without opposition or interdiction. However, the news of the landing spread and Bibulus' fleet quickly mobilised to prevent any further ships from crossing, placing Caesar at a significant numerical disadvantage.

After Caesar's landing, he embarked on a night march against the town of Oricum. His army forced the surrender of the town without a fight; the Pompeian legate in command there – Lucius Manlius Torquatus – was forced by the townspeople to abandon his position.

Bibulus' blockade meant that Caesar was unable to request food from Italy; and although the calendar reported January, the season was late autumn, meaning Caesar would have to wait many months to forage. While some grain ships were present at Oricum, they escaped before Caesar's forces could capture them. He then moved on Apollonia and forced its surrender, before decamping to attack Pompey's main supply centre at Dyrrhachium.

Pompey's reconnaissance was able to detect Caesar's movement toward Dyrrhachium and beat him to the vital supply centre. With Pompey's substantial forces arrayed against him, Caesar withdrew to his already-captured settlements. Caesar called for reinforcements under Mark Antony to transit the Adriatic to support him, but they were interdicted by Bibulus' mobilised fleet; in despair, Caesar attempted to transit from Epirus back to Italy, but was forced back by a winter storm. Pompey's forces, meanwhile, pursued a strategy of starving Caesar's legions out.

However, Antony was able to force a crossing around the time Bibulus died, arriving to Epirus on 10 April with four additional legions. Antony was lucky to escape the Pompeian fleet with minimal losses; Pompey was unable to prevent Antony's reinforcements from joining with Caesar.

Caesar attempted to capture the vital Pompeian logistics hub of Dyrrachium but was unsuccessful after Pompey occupied it and the surrounding heights. In response, Caesar besieged Pompey's camp and constructed a circumvallation thereof, until, after months of skirmishes, Pompey was able to break through Caesar's fortified lines, forcing Caesar to make a strategic retreat into Thessaly.

In a broader sense, the Pompeians elated at the victory, being the first time in the civil war that Caesar had suffered a non-trivial defeat. Men like Domitius Ahenobarbus urged Pompey to bring Caesar to decisive battle and crush him; others urged a return to Rome and Italy to retake the capital. Pompey remained steadfast in believing that committing to a pitched battle was both unwise and unnecessary, deciding on strategic patience to wait for reinforcements from Syria and to exploit Caesar's weak supply lines. The elation of victory turned into overconfidence and mutual suspicion, putting significant pressure on Pompey to provoke a final encounter with the enemy. Beginning to place too much trust in his forces and under the influence of overconfident officers, chose to engage Caesar in Thessaly shortly after being reinforced from Syria.

The siege of Gomphi was a brief military confrontation during Caesar's Civil War. Following defeat at the Battle of Dyrrhachium, the men of Gaius Julius Caesar besieged the Thessalian city of Gomphi. The city fell in a few hours and Caesar's men were allowed to sack Gomphi.

The Battle of Pharsalus was the decisive battle of Caesar's Civil War fought on 9 August 48 BC near Pharsalus in central Greece. Julius Caesar and his allies formed up opposite the army of the Roman Republic under the command of Pompey. Pompey had the backing of a majority of Roman senators and his army significantly outnumbered the veteran Caesarian legions.

Pressured by his officers, Pompey reluctantly engaged in battle and suffered an overwhelming defeat. Pompey, despairing of the defeat, fled with his advisors overseas to Mytilene and thence to Cilicia where he held a council of war; at the same time, Cato and supporters at Dyrrachium attempted first to hand over command to Marcus Tullius Cicero, who refused, deciding instead to return to Italy. They then regrouped at Corcyra and went thence to Libya. Others, including Marcus Junius Brutus sought Caesar's pardon, travelling over marshlands to Larissa where he was then welcomed graciously by Caesar in his camp. Pompey's council of war decided to flee to Egypt, which had in the previous year supplied him with military aid.

In the aftermath of the battle, Caesar captured Pompey's camp and burned Pompey's correspondence. He then announced that he would forgive all who asked for mercy. Pompeian naval forces in the Adriatic and Italy mostly withdrew or surrendered.

According to Caesar, Pompey went from Mytilene to Cilicia and Cyprus. He took funds from the tax collectors, borrowed money to hire soldiers, and armed 2,000 men. He boarded a ship with many bronze coins. Pompey set sail from Cyprus with warships and merchant ships. He heard that Ptolemy was in Pelusium with an army and that he was at war with his sister Cleopatra VII, whom he had deposed. The camps of the opposing forces were close, thus Pompey sent a messenger to announce his arrival to Ptolemy and to request his aid.

Potheinus the eunuch, who was the boy king's regent, held a council with Theodotus of Chios, the king's tutor and Achillas, the head of the army, amongst others. According to Plutarch, some advised driving Pompey away, and others welcoming him. Theodotus argued that neither option was safe: if welcomed, Pompey would become a master and Caesar an enemy, while, if turned away, Pompey would blame the Egyptians for rejecting him and Caesar for making him continue his pursuit. Instead, assassinating Pompey would eliminate fear of him and gratify Caesar.

On 28 September, Achillas went to Pompey's ship on a fishing boat together with Lucius Septimius, who had once been one of Pompey's officers, and a third assassin, Savius. The lack of friendliness on the boat prompted Pompey to tell Septimius that he was an old comrade, the latter merely nodding. He thrust a sword into Pompey, and then Achillas and Savius stabbed him with daggers. Pompey's head was severed, and his unclothed body was thrown into the sea.

When Caesar arrived in Egypt a few days later, he was appalled. He turned away, loathing the man who brought Pompey's head. When Caesar was given Pompey's seal ring, he cried.Theodotus left Egypt and escaped Caesar's revenge. Pompey's remains were taken to Cornelia, who gave them burial at his Alban villa.

Arriving in Alexandria in October 48 BC and seeking initially to apprehend Pompey, his enemy in the civil war, Caesar found that Pompey had been assassinated by Ptolemy XIII's men. Caesar's financial demands and high-handedness then triggered a conflict which put him under siege in Alexandria's palace quarter. Only after external intervention from a Roman client state were Caesar's forces relieved. In the aftermath of Caesar's victory at the Battle of the Nile and Ptolemy XIII's death, Caesar installed his mistress Cleopatra as Egyptian queen, with her younger brother as co-monarch.

The siege of Alexandria was a series of skirmishes and battles occurring between the forces of Julius Caesar, Cleopatra VII, Arsinoe IV, and Ptolemy XIII, between 48 and 47 BC. During this time Caesar was engaged in a civil war against remaining Republican forces. The siege was lifted by relief forces arriving from Syria. After a battle contesting those forces' crossing of the Nile delta, Ptolemy XIII and Arsinoe's forces were defeated.

After defeating Pompey and the optimates at Pharsalus, Julius Caesar pursued his opponents to Asia Minor and then to Egypt. In the Roman province of Asia he left Calvinus in command with an army including the 36th Legion, mainly made up of veterans from Pompey's disbanded legions. With Caesar preoccupied in Egypt and the Roman Republic in the midst of a civil war, Pharnaces saw an opportunity to expand his Kingdom of the Bosphorus into his father's old Pontic empire. In 48 BC he invaded Cappadocia, Bithynia, and Armenia Parva.

Calvinus brought his army to within seven miles of Nicopolis and, avoiding an ambush set by Pharnaces, deployed his army. Pharnaces now retired to the city and awaited a further Roman advance. Calvinus moved his army closer to Nicopolis and built another camp. Pharnaces intercepted a couple of messengers from Caesar requesting reinforcements from Calvinus. He released them hoping the message would cause the Romans to either withdraw or commit to a disadvantageous battle.

Calvinus ordered his men to attack and his lines advanced on the enemy. The 36th defeated their opponents and started to attack the Pontic centre across the trench. Unfortunately for Calvinus, these were the only soldiers in his army to have any success. His recently recruited troops on the left broke and fled after a counterattack. Although the 36th Legion escaped with light losses, just 250 casualties, Calvinus had lost nearly two thirds of his army by the time he had fully disengaged.

The Egyptians had set up camp in a strong position along the Nile, and were accompanied by a fleet. Caesar arrived shortly afterwards, before Ptolemy could attack Mithridates' army. Caesar and Mithridates met 7 miles from Ptolemy's position. In order to reach the Egyptian camp they had to ford a small river. Ptolemy sent a detachment of cavalry and light infantry to stop them from crossing the river. Unfortunately for the Egyptians, Caesar had sent his Gallic and Germanic cavalry to ford the river ahead of the main army. They had crossed undetected. When Caesar arrived he had his men make makeshift bridges across the river and had his army charge the Egyptians. As they did the Gallic and Germanic forces appeared and charged into the Egyptian flank and rear. The Egyptians broke and fled back to Ptolemy's camp, with many fleeing by boat.

Egypt was now in the hands of Caesar, who then lifted the Siege of Alexandria and placed Cleopatra on the throne as co-ruler with another of her brothers, the twelve-year old Ptolemy XIV. Caesar then uncharacteristically lingered in Egypt until April, enjoying a liaison of about two months with the youthful queen before departing to resume his civil war.

News of a crisis in Asia persuaded Caesar to leave Egypt in the middle of 47 BC, at which time sources suggest Cleopatra was already pregnant. He left behind three legions under the command of a son of one of his freedmen to secure Cleopatra's rule. Cleopatra likely bore a child, which she called "Ptolemy Caesar" and which the Alexandrians called "Caesarion", in late June. Caesar believed that the child was his, as he allowed use of the name.

After the defeat of the Ptolemaic forces at the Battle of the Nile, Caesar left Egypt and travelled through Syria, Cilicia and Cappadocia to fight Pharnaces, son of Mithridates VI.

Pharnaces' army marched down into the valley separating the two armies. Caesar was baffled by this move as it meant his opponents had to fight an uphill battle. Pharnaces' men climbed up from the valley and engaged Caesar's thin line of legionaries. Caesar recalled the rest of his men from constructing their camp and hastily drew them up for battle. Meanwhile, Pharnaces' scythed chariots broke through the thin defensive line, but were met by a hail of missiles (pila, the Roman throwing spear) from Caesar's battle line and were forced to retreat. Caesar launched a counter-attack and drove the Pontic army back down the hill, where it was completely routed. Caesar then stormed and took Pharnaces' camp, completing his victory.

It was a decisive point in Caesar's military career - his five-hours campaign against Pharnaces was evidently so swift and complete that, according to Plutarch (writing about 150 years after the battle) he commemorated it with the now famous Latin words reportedly written to Amantius in Rome Veni, vidi, vici ("I came, I saw, I conquered"). Suetonius says that the same three words were displayed prominently in the triumph for the victory at Zela. Pharnaces escaped from Zela, first fleeing to Sinope then back to his Bosporan Kingdom. He started to recruit another army, but was soon after defeated and killed by his son-in-law Asander, one of his former governors who had revolted after the Battle of Nicopolis. Caesar made Mithridates of Pergamum the new king of the Bosporian kingdom in recognition of his aid during the Egyptian campaign.

Caesar ordered his men to gather in Lilybaeum on Sicily in late December. He placed a minor member of the Scipio family – one Scipio Salvito or Salutio – on this staff because of the myth that no Scipio could be defeated in Africa. He assembled six legions there and set out for Africa on 25 December 47 BC. The transit was disrupted by a storm and strong winds; only around 3,500 legionaries and 150 cavalry landed with him near the enemy port of Hadrumentum. Apocryphally, when landing, Caesar fell onto the beach but was able to successfully laugh the bad omen off when he grabbed two handfuls of sand, declaring "I have hold of you, Africa!".

The battle off Carteia was a minor naval battle during the latter stages of Caesar's Civil War won by the Caesarians led by Caesar's legate Gaius Didius against the Pompeians led by Publius Attius Varus. Varus would then join up with the rest of the Pompeians at Munda to meet Caesar. Despite fierce resistance the Pompeians were defeated by Caesar and both Labienus and Varus were killed. 

Titus Labienus commanded the Optimate force and had his 8,000 Numidian cavalry and 1,600 Gallic and Germanic cavalry deploy in unusually close and dense formations for cavalry. The deployment accomplished its goal of misleading Caesar, who believed them to be close-order infantry. Caesar therefore deployed his army in a single extended line to prevent envelopment, with his small force of 150 archers up front and the 400 cavalry on the wings. In a surprising move, Labienus then extended his cavalry on both flanks to envelop Caesar, bringing up his Numidian light infantry in the center. The Numidian light infantry and cavalry began to wear the Caesarian legionaries down with javelins and arrows. This proved very effective, as the legionaries could not retaliate. The Numidians would simply withdraw to a safe distance and continue launching projectiles. The Numidian cavalry routed Caesar's cavalry and succeeded in surrounding his legions, who redeployed into a circle to face attacks from all sides. The Numidian light infantry bombarded the legionaries with missiles. Caesar's legionaries threw their pila at the enemy in return, but were ineffective. The nervous Roman soldiers bunched up together, making themselves easier targets for the Numidian missiles.

Titus Labienus rode up to the front rank of Caesar's troops, coming very near in order to taunt the enemy troops. A veteran of the Tenth Legion approached Labienus, who recognized him. The veteran threw his pilum at Labienus's horse, killing it. "That'll teach you Labienus, that a soldier of the Tenth is attacking you", the veteran growled, shaming Labienus in front of his own men. Some men however began to panic. An aquilifer attempted to flee but Caesar grabbed the man, spun him around and shouted "the enemy are over there!".

Caesar gave the order to make the battle line as long as possible and every second cohort to turn around, so the standards would be facing the Numidian cavalry in the Romans' rear and the other cohorts the Numidian light infantry to the front. The legionaries charged and threw their pila, scattering the Optimates infantry and cavalry. They pursued their enemy for a short distance, and began to march back to camp. However Marcus Petreius and Gnaeus Calpurnius Piso appeared with 1,600 Numidian cavalry and a large number of light infantry who harassed Caesar's legionaries as they retreated. Caesar redeployed his army for combat and launched a counterattack that drove the Optimates forces back over high ground. Petreius was wounded at this point. Completely exhausted, both armies withdrew back to their camps.

The forces of the Optimates, led by Quintus Caecilius Metellus Scipio, were decisively defeated by the veteran forces loyal to Julius Caesar. It was followed shortly by the suicides of Scipio and his ally, Cato the Younger, Numidian King Juba, his Roman peer Marcus Petreius, and the surrender of Cicero and others who accepted Caesar's pardon.

The battle preceded peace in Africa—Caesar pulled out and returned to Rome on July 25 of the same year. However, Caesar's opposition was not done yet; Titus Labienus, the sons of Pompey, Varus and several others managed to gather another army in Baetica in Hispania Ulterior. The civil war was not finished, and the Battle of Munda would soon follow. The Battle of Thapsus is generally regarded as marking the last large scale use of war elephants in the West.

After Caesar's return to Rome, he celebrated four triumphs: over Gaul, Egypt, Asia, and Africa. Caesar, however, left for Spain in November 46 BC, to subdue opposition there. His appointment of Quintus Cassius Longinus after his first campaign in Spain had led to a rebellion: Cassius's "greed and... unpleasant temperament" led to many provincials and troops declaring open defection to the Pompeian cause, in part rallied by Pompey's sons Gnaeus and Sextus. The Pompeians there were joined by other refugees from Thapsus, including Labienus.

After receiving bad news from the peninsula, he left with a single experienced legion, as many of his veterans had been discharged, and put Italy in the hands of his new magister equitum Lepidus. He led eight legions in total, which gave rise to fears that he might be defeated by Gnaeus Pompey's formidable force of more than thirteen legions and further auxiliaries. The Spanish campaign was replete with atrocities, with Caesar treating his enemies as rebels; Caesar's men adorned their fortifications with severed heads and massacred enemy soldiers.

Caesar first arrived in Spain and relieved Ulia from siege. He then marched against Corduba, garrisoned by Sextus Pompey, who requested reinforcements from his brother Gnaeus. Gnaeus at first refused battle at Labienus' advice, forcing Caesar into a winter siege of the city, which was eventually called off after little progress; Caesar then moved to besiege Ategua, shadowed by Gnaeus' army. Substantial desertions, however, started to take their toll on the Pompeian forces: Ategua surrendered on 19 February 45 BC, even after its Pompeian commander massacred suspected defectors and their families on the walls. Gnaeus Pompey's forces retreated from Ategua afterwards, with Caesar following.

The Battle of Munda (17 March 45 BC), in southern Hispania Ulterior, was the final battle of Caesar's civil war against the leaders of the Optimates. With the military victory at Munda and the deaths of Titus Labienus and Gnaeus Pompeius (eldest son of Pompey), Caesar was politically able to return in triumph to Rome, and then govern as the elected Roman dictator. Subsequently, the assassination of Julius Caesar began the Republican decline that led to the Roman Empire, initiated with the reign of the emperor Augustus.

Caesar left his legate Quintus Fabius Maximus to besiege Munda and moved to pacify the province. Corduba surrendered: men in arms present in the town (mostly armed slaves) were executed and the city was forced to pay a heavy indemnity. The city of Munda held out for some time, but, after an unsuccessful attempt to break the siege, surrendered, with 14,000 prisoners taken. Gaius Didius, a naval commander loyal to Caesar, hunted down most of the Pompeian ships. Gnaeus Pompeius looked for refuge on land, but was cornered during the Battle of Lauro and killed.

Although Sextus Pompeius remained at large, after Munda there were no more conservative armies challenging Caesar's dominion. Upon his return to Rome, according to Plutarch, the "triumph which he celebrated for this victory displeased the Romans beyond any thing. For he had not defeated foreign generals, or barbarian kings, but had destroyed the children and family of one of the greatest men of Rome." Caesar was made dictator for life, though his success was short-lived;

The Battle of Lauro (45 BC) was the last stand of Gnaeus Pompeius the Younger, son of Gnaeus Pompeius Magnus, against the followers of Julius Caesar during the civil war of 49–45 BC. After being defeated during the Battle of Munda, the younger Pompeius unsuccessfully attempted to flee Hispania Ulterior by sea, but was eventually forced to land. Pursued by Caesarian forces under Lucius Caesennius Lento, the Pompeians were cornered at a wooded hill near the town of Lauro, where most of them, including Pompeius the Younger, were killed in battle.

Caesar's appointment during the civil war to the dictatorship, first temporarily – then permanently in early 44 BC – along with his de facto and likely indefinite semi-divine monarchical rule, led to a conspiracy which was successful in assassinating him on the Ides of March in 44 BC, three days before Caesar went east to Parthia. Among the conspirators were many Caesarian officers who had rendered excellent service during the civil wars, as well as men pardoned by Caesar.