The Middle Sea: A History of the Mediterranean by John Julius Norwich

Gaul–Caesar being aware that a number of irregularities during his consulship might well render him liable to prosecution if he set foot on Roman territory. There, by dividing the Roman world into three separate spheres of influence–east to Crassus, centre to Caesar, west to Pompey–they decided how best their several ambitions could be fulfilled. Pompey and Crassus would stand for the consulship for the second time in the following year; after that Crassus–who was beginning to feel overshadowed by the other two and was determined to prove himself in battle–would lead an expedition beyond the Euphrates against the Parthian Empire, now the only substantial nation confronting Rome anywhere in the world. Pompey would take over a five-year responsibility for Spain–governing it, however, for most of the time through subordinates so that he could remain in Rome as effective head of the administration. As for Caesar, he would have his Gallic command extended for another five years so that he could extend and consolidate his conquests. 21
    But the partnership’s inevitable stresses and strains were beginning to tell. In 54 BC Julia died in childbirth; she had done much to hold her father and husband together, and with her death they drifted apart. Then, in 53 BC , away in the east, the army of Crassus suffered an overwhelming defeat by Parthian mounted archers at Carrhae (the modern Harran, in southeast Turkey). Of the 6,000 Roman legionaries engaged, 5,500 were killed, and when Crassus went to negotiate peace terms he was killed too. Pompey and Caesar were left alone, each becoming more and more aware of the fact that Rome was not big enough for both of them, and when Pompey rejected Caesar’s suggestion of another marriage tie between their two families, taking instead as his third wife the daughter of Caesar’s enemy Metellus Scipio, whom he then made his fellow consul, it was clear that matters were coming to a head. Pompey, moreover, had the distinct advantage: he was in Rome.
    But Rome was fast declining into anarchy. Although Pompey enjoyed more authority than anyone else, he had almost as many enemies in high places as Caesar, and he was increasingly unable to control the rival gangs of Clodius and his principal adversary, Milo, who divided the streets between them. Then in 52 BC Clodius was murdered and Pompey was made sole consul, with special emergency powers to enable him to restore order in the city; two years later it was moved in the Senate that Caesar should be relieved of his command. The motion was blocked by an energetic young tribune named Curio, one of Caesar’s most ardent supporters, but the stalemate continued. Curio then proposed that both Caesar and Pompey should simultaneously resign their posts, and it was when this proposal too was rejected that one of the consuls then in office called on Pompey to take command of all the forces of the Republic–effectively assuming dictatorial powers. Pompey accepted–on condition, as he put it, that no better way could be found–and immediately took over two legions that happened to be in the capital.
    Curio set off at once with the news to Caesar’s headquarters at Ravenna, then returned to Rome–completing the 140-mile journey in three days–with a letter in which Caesar detailed his immense services to the state and insisted that if he must indeed relinquish his command, Pompey must do the same. The Senate, however, could hardly be persuaded even to have it read; instead they supported a motion by Metellus Scipio (now Pompey’s father-in-law) that Caesar must resign unilaterally or be declared a public enemy. The die, as Caesar himself declared, was cast; and on the night of 10 January 49 BC he and the single legion he had taken with him crossed the little river Rubicon, 22 which constituted the southeastern border of Cisalpine Gaul. In doing so he deliberately flouted the Roman law which forbade a governor to lead an army outside his province,