Pompey was not prepared for the sudden swiftness of his adversary. Without waiting for the reinforcements he had summoned from Gaul, Caesar swooped on Umbria and Picenum, which were not prepared to resist. Town after town surrendered and was won over to his side by the show of clemency and the firm control which Caesar held over his soldiers.
In six weeks he was joined by another legion from Gaul. Corfinium was surrendered to him and he sped south in pursuit of Pompey.
The legions Pompey had ready were the very legions which Caesar had led to victory in Gaul. Pompey hence could not rely on the loyalty of his troops. Instead he decided to move south to the port of Brundisium where he embarked with his troops and sailed east, hoping to raise troops there with which he could return to drive the rebel out of of Italy. His leaving words are said to have been "Sulla did it, why not I ?"
Caesar, with no enemy left to fight in Italy, was in Rome no longer than three months after he had crossed the river Rubicon.
He immediately secured the treasury and then, rather than pursuing Pompey, he turned west to deal with the legions in Spain who were loyal to Pompey.
The campaign in Spain was not a series of battles, but a sequence of skillful manouvers by both sides - during which Caesar, by his own admission, was at times outgeneraled by his opposition. But Caesar remained the winner as within six months most of the Spanish troops had joined his side.
Returning to Rome he became dictator, passed popular laws, and then prepared for the decisive contest in the east, where a large force was now collecting under Pompey.
Pompey also controlled the seas, as most of the fleet had joined with him. Caesar therefore managed only with great difficulty to set across to Epirus with his first army. There he was shut up, unable to manoeuvre, by the much larger army of Pompey. With even more difficulty his lieutenant, Mark Antony, joined him with the second army in the spring of 48 BC.
Some months of manoeuvring following Pompey, though his forces outnumbered Caesar's, knew well that his eastern soldiers were not to be matched against Caesar's veterans. Hence he wished to avoid a pitched battle. Many of the senators though, who had fled Italy together with Pompey, scoffed at his indecision and clamoured for battle.
Until at last, in midsummer, Pompey was goaded into delivering an attack on the plain of Pharsalus in Thessaly.
The fight hung long in balance, but eventually ended in the complete rout of Pompey's army, with immense slaughter. Most of the Romans on Pompey's side though were persuaded by Caesar's promises of clemency to surrender once they realized the battle lost.
Pompey himself escaped to the coast, took a ship with a few loyal comrades and made his way to Egypt, where he found awaiting him not the asylum he sought, but the dagger of an assassin commissioned by the Egyptian government.