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 want for another consulship was delaying 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, leading one legion, the Legio XIII Gemina, General Julius Caesar crossed the Rubicon River, the boundary between the Cisalpine Gaul province to the north and Italy proper to the south, a legally proscribed action forbidden to any army-leading general. The proscription protected the Roman Republic from a coup d'état; thus, Caesar's military action began a civil war.
This act of war on the Roman Republic by Caesar led to widespread approval amongst the Roman civilians, who regarded him as a hero. 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 progress. 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 disposed of at least two legions: some 11,500 soldiers and some hastily levied Italian troops commanded by Lucius Domitius Ahenobarbus (Domitius). 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 make junction 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 (the Optimates)—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; to wit, 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 River frontier 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, in Roman Greece.
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 (Afranius) and Marcus Petreius (Petreius), afterwards pacifying Roman Hispania; in campaign, the Caesarian forces—six legions, 3,000 cavalry (Gallic campaign veterans), and Caesar's 900-horse personal bodyguard—suffered 70 men killed in action, while the Pompeian forces lost 200 men killed and 600 wounded.
Returned 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 pursuit of Pompey, then in Roman Greece.
From Brundisium, Caesar with 7 legions crossed the Strait of Otranto to the gulf of Valona, not Palaesta in Epirus (modern Palase/Dhermi, Albania), as reported by Lucan. In that time, Pompey considered three courses of action: (i) alliance with the King of Parthia, an erstwhile ally, far to the east; (ii) invade Italy with his naval superiority; and (iii) confronting Julius Caesar in decisive battle. A Parthian alliance was unfeasible, 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 councilor's advice, Pompey decided to fight Julius Caesar in decisive battle.
Moreover, Caesar's pursuing him to Illyrium, across the Adriatic Sea, decided the matter, and, on 10 July 48 BC, Pompey fought him in the Battle of Dyrrhachium, costing Caesar 1,000 veteran legionaries and a retreat. 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, definitive coup de grâce, thus losing the initiative, and the chance to quickly conclude Caesar's Civil War; meanwhile, Caesar retreated southwards. Near Pharsalus, Caesar pitched a strategic bivouac, and Pompey attacked, yet, despite his much larger army, 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 they camped and became involved with the Alexandrine civil war between Ptolemy and his sister, wife, and co-regnant queen, the Pharaoh 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 Romans being distracted by their civil war to oppose the Roman-friendly Deiotarus and make himself the ruler of Colchis and lesser Armenia. At Nicopolis Pharnaces had defeated what little Roman opposition Caesar's lieutenant, 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 against the Romans, Pharnaces drew back to suppress revolt in 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; Caesar's speed brought war quickly and battle took place near Zela (modern Zile in Turkey), where Pharnaces was routed 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; Dio Cassius says Pharnaces was captured and then killed.
Caesar returned to Rome to deal with several mutinous legions. While Caesar had been in Egypt installing Cleopatra as Queen, 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.49 BC
January 1: The Roman Senate receives a proposal from Julius Caesar that he and Pompey should lay down their commands simultaneously. The Senate responds that Caesar must immediately surrender his command.
January 10: Julius Caesar leads his army across the Rubicon, which separates his jurisdiction (Cisalpine Gaul) from that of the Senate (Italy), and thus initiates a civil war.
February, Pompey's flight to Epirus (in Western Greece) with most of the Senate
March 9, Caesar's advance against Pompeian forces in Hispania
April 19, Caesar's siege of Massilia against the Pompeian Lucius Domitius Ahenobarbus, later the siege was conducted by Caesarian Gaius Trebonius
June, Caesar's arrival in Hispania, where he was able to seize the Pyrenees passes defended by the Pompeian L. Afranius and M. Petreius.
July 30, Caesar surrounded Afranius and Petreius's army in Ilerda
August 2, Pompeians in Ilerda surrendered to Caesar
August 24: Caesar's general Gaius Scribonius Curio, is defeated in North Africa by the Pompeians under Attius Varus and King Juba I of Numidia (whom he defeated earlier in the Battle of Utica, in the Battle of the Bagradas River), and commits suicide.
September Decimus Junius Brutus Albinus, a Caesarian, defeated the combined Pompeian-Massilian naval forces in the naval Battle of Massilia, while the Caesarian fleet in the Adriatic was defeated near Curicta (Krk)
September 6, Massilia surrendered to Caesar, coming back from Hispania
October, Caesar appointed Dictator in Rome; presides over his own election as consul and resigns after eleven days
January 4, Caesar landed at Dyrrhachium (Durazzo)
March, Antony joined Caesar
April, Battle of Dyrrhachium
July 10: Battle of Dyrrhachium, Julius Caesar barely avoids a catastrophic defeat by Pompey in Macedonia, he retreats to Thessaly.
August 9: Battle of Pharsalus: Julius Caesar decisively defeats Pompey at Pharsalus and Pompey flees to Egypt.
September 28, Caesar learned that Pompey was assassinated.
Siege of Alexandria
October, Pharnaces, King of Bosporus defeated the Caesarian Gnaeus Domitius Calvinus in the Battle of Nicopolis (or Nikopol)
December: Battle in Alexandria, Egypt between the forces of Caesar and his ally Cleopatra VII of Egypt and those of rival King Ptolemy XIII of Egypt and Queen Arsinoe IV. The latter two are defeated and flee the city; Cleopatra becomes queen of Egypt. During the battle part of the Library of Alexandria catches fire and is burned down.
Caesar is named Dictator for one year.
February: Caesar and his ally Cleopatra defeat the forces of the rival Egyptian Queen Arsinoe IV in the Battle of the Nile, Ptolemy was killed, Caesar then relieved his besieged forces in Alexandria
May: Caesar defeated Pharnaces II of Pontus, king of the Bosporus in the Battle of Zela. (This is the war that Caesar tersely described veni, vidi, vici.)
Pharaoh Cleopatra VII of Egypt promotes her younger brother Ptolemy XIV of Egypt to co-ruler.
August, Caesar quelled a mutiny of his veterans in Rome.
October, Caesar's invasion of Africa, against Metellus Scipio and Labienus, Caesar's former lieutenant in Gaul
January 4: Caesar narrowly defeats his former second in command Titus Labienus in the Battle of Ruspina; nearly 1/3 of Caesar's army is killed.
February 6: Caesar defeats the combined army of Pompeian followers and Numidians under Metellus Scipio and Juba in the Battle of Thapsus. Cato commits suicide. Afterwards, he is accorded the office of Dictator for the next ten years.
November: Caesar leaves for Farther Hispania to deal with a fresh outbreak of resistance.
Caesar, in his role as Pontifex Maximus, reforms the Roman calendar to create the Julian calendar. The transitional year is extended to 445 days to synchronize the new calendar and the seasonal cycle. The Julian Calendar would remain the standard in the western world for over 1600 years, until superseded by the Gregorian Calendar in 1582.
Caesar appoints his grandnephew Gaius Octavius his heir.
January 1: Julian calendar goes into effect
March 17: In his last victory, Caesar defeats the Pompeian forces of Titus Labienus and Pompey the younger in the Battle of Munda. Pompey the younger was executed, and Labienus died in battle, but Sextus Pompey escaped to take command of the remnants of the Pompeian fleet.
The veterans of Caesar's Legions Legio XIII Gemina and Legio X Equestris demobilized. The veterans of the 10th legion would be settled in Narbo, while those of the 13th would be given somewhat better lands in Italia itself.
Caesar probably writes the Commentaries in this year
Julius Caesar is named Dictator perpetuo ("dictator in perpetuity")
Julius Caesar is assassinated on March 15, the Ides of March.
Caesar was later proclaimed dictator first for ten years and then in perpetuity. The latter arrangement in openly doing away with a term limit, triggered the conspiracy leading to his assassination on the Ides of March in 44 BC out of such fears. Following this, Antony and Caesar's adopted son Octavius (later Caesar Augustus) had to fight yet another civil war against remnants of the Optimates and Liberatores faction, who were crushed by the skill of Marcus Antonius, who was able to defeat his two main opponents, and Octavius, despite having his camp overrun, evaded capture.