Cicero returned from banishment an altered man. Though his return had been glorious, he saw that his position was entirely changed, and he was forced to yield to a power which he no longer dared to resist. He even lent his support to the Triumvirs, and praised in public those proceedings which he had once openly and loudly condemned. Meantime the power of Pompey had been shaken at Rome. A misunderstanding had sprung up between him and Crassus, and Cato and the other leaders of the aristocracy attacked him with the utmost vehemence. The Senate began to entertain hopes of recovering their power. They determined to support L. Domitius Ahenobarbus, who, in B.C. 56, had become a candidate for the Consulship for the following year, and who threatened to deprive Cæsar of his provinces and armies. Under these circumstances Cæsar invited Pompey and Crassus to meet him at Luca (Lucca) in the spring of B.C. 56. He reconciled them to each other, and arranged that they were to be Consuls for the next year, and obtain provinces and armies, while he himself was to have his government prolonged for another five years, and to receive pay for his troops. On their return to Rome, Pompey and Crassus became candidates for the Consulship; but Domitius Ahenobarbus, supported by Cato and the aristocracy, offered a most determined opposition. The Consul Lentulus Marcellinus likewise was resolved to use every means to prevent their election; and, finding it impossible to carry their election while Marcellinus was in office, they availed themselves of the veto of two of the Tribunes to prevent the Consular Comitia from being held this year. The elections, therefore, did not take place till the beginning of B.C. 55, under the presidency of an interrex. Even then Ahenobarbus and Cato did not relax in their opposition; and it was not till the armed bands of Pompey and Crassus had cleared the Campus Martius of their adversaries that they were declared Consuls for the second time (B.C. 55).
They forthwith proceeded to carry into effect the compact that had been made at Luca. They induced the Tribune C. Trebonius to bring forward two bills, one of which gave the province of the two Spains to Pompey, and that of Syria to Crassus; the other prolonged Cæsar's government for five years more, namely, from the 1st of January, B.C. 53, to the end of the year 49. Pompey was now at the head of the state; and at the expiration of his year of office would no longer be a private man, but with the command of an army and in possession of the imperium. With an army he felt sure of regaining his former influence. He had now completed the theatre which he had been some time building, and, as a means of regaining the popular favor, he resolved to open it with an exhibition of games of unparalleled splendor and magnificence. The building itself was worthy of the conqueror of the East. It was the first stone theatre that had been erected at Rome, and was sufficiently large to accommodate 40,000 spectators. The games exhibited lasted many days. Five hundred African lions and eighteen elephants were killed. A rhinoceros was likewise exhibited on this occasion for the first time. Pompey sent an army into Spain under the command of his lieutenants, L. Afranius and M. Petreius, while he himself remained in the neighborhood of Rome as Proconsul.
Before the end of the year Crassus set out for Syria, with the intention of attacking the Parthians. He was anxious to distinguish himself in war, like Pompey and Cæsar, and, though upward of sixty years of age, he chose rather to enter upon an undertaking for which he had no genius than to continue the pursuit of wealth and influence at home. He crossed the Euphrates in B.C. 54, but, hesitating to proceed at once against Parthia, he gave the enemy time to assemble his forces, and returned to Syria without accomplishing any thing of importance. He spent the winter in Syria, where, instead of exercising his troops and preparing for the ensuing campaign, he plundered the temples, and employed his time in collecting money from every quarter. In the following spring (B.C. 53) he again crossed the Euphrates, and plunged into the sandy deserts of Mesopotamia. He trusted to the guidance of an Arabian chieftain, who promised to lead him by the shortest way to the enemy. But this man was in the pay of Surenas, the Parthian general; and when he had brought the Romans into the open plains of Mesopotamia, he seized a frivolous pretext, and rode off to inform Surenas that the Roman army was delivered into his hands. The Parthians soon appeared. They worried the densely-marshaled Romans with showers of arrows; and by feigned retreats, during which they continued to discharge their arrows, they led the Romans into disadvantageous positions. The son of Crassus, who had distinguished himself as one of Cæsar's lieutenants in Gaul, was slain, and the Romans, after suffering great loss, retreated to Carrhæ, the Haran of Scripture. On the following day they continued their retreat; and Surenas, fearing that Crassus might after all make his escape, invited him to an interview. He was treacherously seized, and, in the scuffle which ensued, was slain by some unknown hand. His head was carried to the Parthian king Orodes, who caused melted gold to be poured into the mouth, saying, "Sate thyself now with that metal of which in life thou wert so greedy" Twenty thousand Roman troops were slain, and ten thousand taken prisoners, in this expedition, one of the most disastrous in which the Romans were ever engaged. Only a small portion of the Roman army escaped to Syria under the command of L. Cassius Longinus, afterward one of Cæsar's assassins, who had displayed considerable ability during the war, but whose advice Crassus had constantly refused to follow.
The death of Crassus left Pompey and Cæsar alone at the head of the state, and it became evident that sooner or later a struggle would take place between them for the supremacy. The death of Julia, in B.C. 54, to whom both her father and husband were strongly attached, broke a link which might have united them much longer. Pompey considered that he had been the chief means of raising Cæsar to power, and he appeared long to have deemed it impossible that the conqueror of Mithridates could be thrown into the shade by any popular leader. Such a result, however, was now imminent. Cæsar's brilliant victories in Gaul were in every body's mouth, and Pompey saw with ill-disguised mortification that he was becoming the second person in the state. Though this did not lead him to break with Cæsar at once, it made him anxious to increase his power and influence, and he therefore now resolved, if possible, to obtain the Dictatorship. He accordingly used no effort to put an end to the disturbances at Rome between Milo and Clodius in this year, in hopes that all parties would be willing to accede to his wishes in order to restore peace to the city. Milo was a candidate for the Consulship and Clodius for the Prætorship. Each was attended by a band of hired ruffians; battles took place between them daily in the Forum and the streets; all order and government were at an end. In such a state of things no elections could be held, and the confusion at length became downright anarchy, when Milo murdered Clodius on the 20th of January in the following year (B.C. 52). The two rivals had met near Bovillæ, accompanied, as usual, by their armed followers. A fray ensued. The party of Milo proved the stronger, and Clodius took refuge in a house. But Milo attacked the house, dragged out Clodius, and having dispatched him, left him dead upon the road. His body was found by a Senator, carried to Rome, and exposed naked to the people. They were violently excited at the sight, and their feelings were still farther inflamed by the harangues of the Tribunes. The benches and tables of the Senate-house were seized to make a funeral pile for their favorite; and not only the Senate-house, but several other public buildings, were reduced to ashes. As the riots still continued, the Senate had no longer any choice but to call in the assistance of Pompey. They therefore commissioned him to collect troops and put an end to the disturbances. Pompey, who had obtained the great object of his desires, obeyed with alacrity; he was invested with the supreme power of the state by being elected sole Consul on the 25th of February; and, in order to deliver the city from Milo and his myrmidons, he brought forward laws against violence and bribery at elections. Milo was put upon his trial; the court was surrounded with soldiers; Cicero, who defended him, was intimidated, and Milo was condemned, and went into exile at Massilia. Others shared the same fate, and peace was once more restored to the state.
Pompey's jealousy of Cæsar brought him into connection with the aristocratical party. After Julia's death he had married Cornelia, the daughter of Metellus Scipio, whom he made his colleague on the first of August. His next step was to strike a blow at Cæsar. He brought forward an old law that no one should become a candidate for a public office while absent, in order that Cæsar might be obliged to resign his command, and to place himself in the power of his enemies at Rome, if he wished to obtain the Consulship a second time. But the renewal of this enactment was so manifestly aimed at Cæsar that his friends insisted he should be specially exempted from it; and as Pompey was not yet prepared to break openly with him, he thought it more expedient to yield. At the same time, Pompey provided that he himself should remain in command of an army after his rival had ceased to have one, by obtaining a senatus consultum, by which his government of the Spains was prolonged for another five years. And, in case Cæsar should obtain the Consulship, he caused a law to be enacted, in virtue of which no one could have a province till five years had elapsed from the time of his holding a public office. Such were the precautions adopted against Cæsar, the uselessness of which time soon showed.
In the following year (B.C. 51) Pompey declared himself still more openly on the side of the Senate; but still he shrank from supporting all the violent measures of the Consul M. Claudius Marcellus, who proposed to send a successor to Cæsar, on the plea that the war in Gaul was finished, and to deprive him of the privilege of becoming a candidate for the Consulship in his absence. The Consuls for the next year (B.C. 50), L. Æmilius Paullus and C. Claudius Marcellus, and the powerful Tribune C. Curio, were all reckoned devoted partisans of Pompey and the Senate. Cæsar, however, gained over Paullus and Curio by large bribes, and with a lavish hand distributed immense sums of money among the leading men of Rome. It was proposed in the Senate by the Consul C. Marcellus that Cæsar should lay down his command by the 13th of November. But this was an unreasonable demand; Cæsar's government had upward of another year to run; and if he had come to Rome as a private man to sue for the Consulship, there can be no doubt that his life would have been sacrificed. Cato had declared that he would bring Cæsar to trial as soon as he laid down his command; but the trial would have been only a mockery, for Pompey was in the neighborhood of the city at the head of an army, and would have overawed the judges by his soldiery as at Milo's trial. The Tribune Curio consequently interposed his veto upon the proposition of Marcellus. The Senate, anxious to diminish the number of his troops, had, under pretext of a war with the Parthians, ordered that Pompey and Cæsar should each furnish a legion to be sent into the East. The legion which Pompey intended to devote to this service was one he had lent to Cæsar in B.C. 53, and which he now accordingly demanded back; and, although Cæsar saw that he should thus be deprived of two legions, which would probably be employed against himself, he complied with the request. Upon their arrival in Italy, they were not sent to the East, but were ordered to pass the winter at Capua. Cæsar took up his quarters at Ravenna, the last town in his province bordering upon Italy.
Though war seemed inevitable, Cæsar still showed himself willing to enter into negotiations with the aristocracy, and accordingly sent Curio with a letter addressed to the Senate, in which he expressed his readiness to resign his command if Pompey would do the same. Curio arrived at Rome on the 1st of January, B.C. 49, the day on which the new Consuls, L. Cornelius Lentulus and C. Claudius Marcellus, entered upon their office. It was with great difficulty that the Tribunes, M. Antonius, afterward the well-known Triumvir, and Q. Cassius Longinus, forced the Senate to allow the letter to be read. After a violent debate, the motion of Scipio, Pompey's father-in-law, was carried, "that Cæsar should disband his army by a certain day, and that if he did not do so he should be regarded as an enemy of the state" On the 6th of January the Senate passed the decree investing the Consuls with dictatorial power. Antonius and Cassius, considering their lives no longer safe, fled from the city in disguise to Cæsar's army, and called upon him to protect the inviolable persons of the Tribunes. This was the crisis. The Senate intrusted the management of the war to Pompey, determined that fresh levies of troops should be held, and voted a sum of money from the public treasury to Pompey. Pompey all along had no apprehensions as to the war; he thought it impossible that Cæsar should ever march against him; he was convinced that his great fame would cause a multitude of troops to flock around him whenever he wished. In addition to this, he had been deceived as to the disposition of Cæsar's troops: he had been led to believe that they were ready to desert their general at the first opportunity. Consequently, when the war broke out, Pompey had scarcely any troops except the two legions which he had obtained from Cæsar, and on the fidelity of which he could by no means rely.