After the "crossing of the Rubicon," things went bad for Pompey and the optimates almost immediately. Caesar only had one legion at first, divided into two groups, but those two groups moved with Caesar's typical swiftness and surprised everyone. He himself immediately moved to take control of the vital route at the head of the Via Flamminia, and sent Antonius to block the via Cassia at Arretium. Both of these goals were accomplished within the first six days of the conflict, between January 10 and 16 of 49 BCE. When news of this reached Rome on the 7th day, Pompey immediately quit the city and tried to reorganize in Campania. Our sources for this are Caesar's own accounts, as well as scattered corroboration via Cicero's private correspondence.

The proconsul who was to replace Caesar in Gallia Transalpina, Domitius Ahenobarbus, did not help matters: he had cobbled together the equivalent of roughly three legions and tried to make a stand against Caesarian forces at the important crossroads of Corfinium, but the balance of Caesar's Gallic legions were now moving into Italia, quickly overruning Picenum, and they surrounded Ahenobarbus. His men forced him to surrender, and then many of them joined Caesar's side. Pompey had requested (demanded?) that he move his legions south to join him in Campania, but Ahenobarbus ignored him (they were technically of equal rank, both having imperium proconsularis). Those three legions were absolutely essential, especially as many of them had been recruited in Picenum, the land from which Pompey boasted that he could raise loyal legionnaires simply by stomping his foot. From then on it was a steady retreat for Pompey and his scattered forces, such as they were. By late February, all the opposition to Caesar was hemmed in at Brundisium and preparing to make the jump to Greece, and Caesar approaching the port fast with three veteran legions and at least three more of new recruits.

Pompey's decision to abandon first the city, then the whole of the Italian peninsula, was no doubt unpopular but was probably militarily sound. Two of the legions with him in Italia were "on loan" from Caesar, technically, and he didn't trust them. His veterans were aging, and had not seen combat in some time (compared to Caesar's grizzled legions, the veterans of nine years of hard fighting in Gaul). The rest of Pompey's forces were raw recruits, which Domitius Ahenobarbus had discovered to be very untrustworthy in this conflict. In the preceding years, Caesar had been busy cozying (cozing?) up to the cities of Italy, especially in the north, or so it seems from what he tells us about how they eagerly opened their gates to Caesarian forces. Pompey rightly felt his position to be rather weak after the initial surge, and further eroded by the incredible speed of Caesar's move into Italia. He was a veteran of military matters and no fool, and would not risk a pitched battle in Italy on uncertain terms when he was confident that, with time, he could raise and train a superior force in the East. Cicero and no doubt many of the optimates disagreed and thought the loss of the capital, and then all of Italy, was disastrous. Certainly once Pompey's forces fled, Caesar was free to enter the capital and arrange things favorably for himself--history is written by the victors, and the citizens of Rome were not in any position to question Caesar's versions of events. In addition, Caesar's forces moved under the unhesitating will of a single man, while Pompey was saddled with a veritable gaggle of arm-chair-strategist senators who seem to second-guess him at every turn, including Cicero. The standard rhetoric is that Pompey had grossly misjudged the recruiting potential and resiliency of his base in Picenum, and had not anticipated the speed of Caesar's move, especially with only a single legion at the outset. I think both of these notions are probably true to some extent, despite our biased (Caesar) and spotty (Cicero) sources on this crucial period.