Caesar's timing was far-sighted: while Pompey's forces actually vastly outnumbered Caesar's single legion, composing at least 100 cohorts, or 10 legions, "by no stretch of the imagination could Italy have been described as prepared to meet an invasion". Caesar captured Ariminum (modern day Rimini) without resistance, his men having already infiltrated the city; he captured three more cities in quick succession.

In late January, Caesar and Pompey were negotiating, with Caesar proposing that the two of them return to their provinces (which would have required Pompey to travel to Spain) and then disband their forces. Pompey accepted those terms provided that they withdraw from Italy at once and submit to arbitration of the dispute by the Senate, a counter-offer that Caesar rejected as doing so would have put him at the mercy of hostile senators while giving up all the advantages of his surprise invasion. Caesar continued to advance.

After encountering five cohorts under Quintus Minucius Thermus at Iguvium, Thermus' forces deserted. Caesar quickly overran Picenum, the area from which Pompey's family originated. While Caesar's troops skirmished once with local forces, fortunately for him, the population was not hostile: his troops were refraining from looting and his opponents had "little popular appeal". In February 49 BCE, Caesar received reinforcements and captured Asculum when the local garrison deserted.