Early in 52 B.C. some Roman traders were massacred at Cenabum (Orléans), and, on hearing the news, the Arverni revolted under Vercingetorix and were quickly joined by other tribes, especially the Bituriges, whose capital was Avaricum (Bourges). Caesar hastened back from Italy, slipped past Vercingetorix and reached Agedincum (Sens), the headquarters of his legions. Vercingetorix saw that Caesar could not be met in open battle, and determined to concentrate his forces in a few strong positions. Caesar first besieged and took Avaricum, whose occupants were massacred, and then invested Gergovia (near the Puy-de-Dôme), the capital of the Arverni, but suffered a severe repulse and was forced to raise the siege. Hearing that the Roman province was threatened, he marched westward, defeated Vercingetorix near Dijon and shut him up in Alesia (Mont-Auxois), which he surrounded with lines of circumvallation. An attempt at relief by Vercassivellaunus was defeated after a desperate struggle and Vercingetorix surrendered. The struggle was over except for some isolated operations in 51 B.C., ending with the siege and capture of Uxellodunum (Puy d'Issolu), whose defenders had their hands cut off.

Caesar now reduced Gaul to the form of a province, fixing the tribute at 40,000,000 sesterces (£350,000), and dealing liberally with the conquered tribes, whose cantons were not broken up.

In the meantime his own position was becoming critical. In 56 B.C., at the conference of Luca (Lucca), Caesar, Pompey Break-up of the Coalition. and Crassus had renewed their agreement, and Caesar's command in Gaul, which would have expired on the 1st of March 54 B.C., was renewed, probably for five years, i.e. to the 1st of March 49 B.C., and it was enacted that the question of his successor should not be discussed until the 1st of March 50 B.C., by which time the provincial commands for 49 B.C. would have been assigned, so that Caesar would retain imperium, and thus immunity from persecution, until the end of 49 B.C. He was to be elected consul for 48 B.C., and, as the law prescribed a personal canvass, he was by special enactment dispensed from its provisions. But in 54 B.C. Julia, the daughter of Caesar and wife of Pompey, died, and in 53 B.C. Crassus was killed at Carrhae. Pompey now drifted apart from Caesar and became the champion of the senate. In 52 B.C. he passed a fresh law de jure magistratuum which cut away the ground beneath Caesar's feet by making it possible to provide a successor to the Gallic provinces before the close of 49 B.C., which meant that Caesar would become for some months a private person, and thus liable to be called to account for his unconstitutional acts.

Caesar had no resource left but uncompromising obstruction, which he sustained by enormous bribes. His representative in 50 B.C., the tribune C. Scribonius Curio, served him well, and induced the lukewarm majority of the senate to refrain from extreme measures, insisting that Pompey, as well as Caesar, should resign the imperium. But all attempts at negotiation failed, and in January 49 B.C., martial law having been proclaimed on the proposal of the consuls, the tribunes Antony and Cassius fled to Caesar, who crossed the Rubicon (the frontier of Italy) with a single legion, exclaiming "Alea jacta est."

Pompey's available force consisted in two legions stationed in Campania, and eight, commanded by his lieutenants, Afranius The Civil war and Petreius, in Spain; both sides levied troops in Italy. Caesar was soon joined by two legions from Gaul and marched rapidly down the Adriatic coast, overtaking Pompey at Brundisium (Brindisi), but failing to prevent him from embarking with his troops for the East, where the prestige of his name was greatest. Hereupon Caesar (it is said) exclaimed "I am going to Spain to fight an army without a general, and thence to the East to fight a general without an army." He carried out the first part of this programme with marvellous rapidity. He reached Ilerda (Lerida) on the 23rd of June and, after extricating his army from a perilous situation, outmanoeuvred Pompey's lieutenants and received their submission on the 2nd of August. Returning to Rome, he held the dictatorship for eleven days, was elected consul for 48 B.C., and set sail for Epirus at Brundisium on the 4th of January. He attempted to invest Pompey's lines at Dyrrhachium (Durazzo), though his opponent's force was double that of his own, and was defeated with considerable loss.

He now marched eastwards, in order if possible to intercept the reinforcements which Pompeys father-in-law, Scipio, was bringing up; but Pompey was able to effect a junction with this force and descended into the plain of Thessaly, where at the battle of Pharsalus he was decisively defeated and fled to Egypt, pursued by Caesar, who learnt of his rival's murder on landing at Alexandria. Here he remained for nine months, fascinated (if the story be true) by Cleopatra, and almost lost his life in an émeute. In June 47 B.C. he proceeded to the East and Asia Minor, where he "came, saw and conquered" Pharnaces, son of Mithradates the Great, at Zela. Returning to Italy, he quelled a mutiny of the legions (including the faithful Tenth) in Campania, and crossed to Africa, where a republican army of fourteen legions under Scipio was cut to pieces at Thapsus (6th of April 46 B.C.). Here most of the republican leaders were killed and Cato committed suicide. On the 26th to 29th July Caesar celebrated a fourfold triumph and received the dictatorship for ten years. In November, however, he was obliged to sail for Spain, where the sons of Pompey still held out.

On the 17th of March 45 B.C. they were crushed at Munda. Caesar returned to Rome in September, and six months later (15th of March 44 B.C.) was murdered in the senate house at the foot of Pompey's statue.

It was remarked by Seneca that amongst the murderers of Caesar were to be found more of his friends than of his enemies. Caesar's dictatorship We can account for this only by emphasizing the fact that the form of Caesar's government became as time went on more undisguised in its absolutism, while the honours conferred upon seemed designed to raise him above the rest of humanity. It is explained elsewhere (see Rome: History, Ancient) that Caesar's power was exercised under the form of dictatorship. In the first instance (autumn of 49 B.C.) this was conferred upon him as the only solution of the constitutional deadlock created by the flight of the magistrates and senate, in order that elections (including that of Caesar himself to the consulship) might be held in due course. For this there were republican precedents. In 48 B.C. he was created dictator for the second time, probably with constituent powers and for an undefined period, according to the dangerous and unpopular precedent of Sulla. In May 46 B.C. a third dictatorship was conferred on Caesar, this time for ten years and apparently as a yearly office, so that he became Dictator IV. in May 45 B.C. Finally, before the 15th of February 44 B.C., this was exchanged for a life-dictatorship. Not only was this a contradiction in terms, since the dictatorship was by tradition a makeshift justified only when the state had to be carried through a serious crisis, but it involved military rule in Italy and the permanent suspension of the constitutional guarantees, such as intercessio and provocatio, by which the liberties of Romans were protected.