Roman triumvir, general and politician
The life of Gnaeus Pompeius Magnus (usually anglicised to Pompey) has been overshadowed by history. In hindsight, he comes across as an almost literary antagonist for his great rival Gaius Iulius Caesar. He's the opponent to be defeated at the climax of the action, before the tragic denouement on the Senate floor, before the knives and the blood and the betrayal.
It is true that Pompey and Caesar were almost destined to become rivals. Pompey was born in 106 BC, 6 years before Caesar; as age-mates, they would compete for miltary and political offices throughout their lives. When they were young, they were on opposing sides in the great civil conflict of their parents' generation. As a nephew of Marius, Caesar barely escaped with his life from the collapse of his uncle's cause. Pompey, meanwhile, raised three legions in support of Marius' rival Sulla in 84 BC.
It was Sulla who first gave Pompey his cognomen, Magnus ("the Great"), after a series of military victories in the aftermath of the civil war. The reference to Alexander the Great was deliberate, but sardonic. In later life, he would build on his cognomen, adopting as much of the iconography of Alexander the Great as he could.
After Sulla's death in 78 BC, Pompey continued his miltary career. He helped put down a rebellion in Spain from 76 - 71 BC, then joined M. Licinius Crassus in the war against Spartacus in late 71. All this miltary activity had a politcal reward when he was elected consul in 70 BC, at the young age of 36.
By 67, Rome needed his military talents again. Piracy, always a problem in the ancient Mediterranean, had become so bad that communication was blocked between Rome and its colonies in Spain and Africa. Pompey was given supreme command against the pirates, and restored communications within forty days. By the end of three months, he had entirely eliminated the problem.
His next chance to show his military skill was in Asia Minor. One of Rome's client kings, Mithridates, was attempting to break away from Roman rule. Pompey routed Mithridates and converted much of his territory into Roman colonies. As was standard practice, he then returned to Rome, disbanded his army, and asked the Senate to ratify his actions.
The Sentate refused.
By this time, both Pompey and Caesar had been seriously opposed by the Senate. Along with Crassus, they formed an alliance designed to act as a counterweight to senatorial power: the first triumvirate. It was a private arrangement rather than an official one, cemented in part by Pompey's marriage to Caesar's daughter Julia.
Under the arrangement, Pompey was to stay in Rome while Caesar fought in Gaul. Despite his absence, Caesar continued to gain popularity in the Senate, until Pompey was no longer considered his equal. Julia died in 54 and Crassus in 53, and Pompey felt that the triumvirate had outlived its usefulness.
Knowing that, if Caesar were allowed back into Rome and given a triumph for his conquest of Gaul, then his rival would effectively control the city, Pompey agitated against him in the Senate. Caesar was commanded to disband his army and return to Rome unarmed. He refused, and crossed the Rubicon with his troops.
The inevitable conflict came. Each was struggling for the upper hand, for the favor of the Senate, for the backing of the armies. each knew that, if he lost, his life would be forfeit. It was a bitter civil war, dividing an entire generation. Initially, Pompey had the upper hand, but lost it when he left Rome to pursue Caesar. In 48 BC, at the battle of Pharsalus, he suffered a disastrous defeat.
Pompey fled to Egypt, but was murdered on landing. His head was pickled in brine and sent to Caesar. Pompey's sons attempted to contiune the war (ironically, by piracy, among other tactics), but their attempts were short-lived.
As a soldier, Pompey was at least Caesar's equal. He was more fastidious in his person, more scrupulous in his honor, and less subtle in public relations. This last point, of course, is the telling one. His attempts to portray himself as a second Alexander the Great come across as clumsy and contrived, while Caesar's description of the Gallic Wars remains one of the best examples of self publicity in history.
The rivalry between Pompey and Caesar begs the question: what would have happened had Pompey won? Would he, too, have been installed dictator for life? Would he, or some successor of his, have done what Augustus did, destroying the Republic to save Rome?
Judging by what we know, Pompey was a staunch constitutionalist. Although he joined the triumvirate, which was an extra-legal arrangement, he seems to have truly believed in the laws of the Republic. Even if he had accepted a dictatorship, it is unlikely that he would have left anyone in the position Caesar left Octavian, with the means and the will to take over Rome.