After his defeat at Dyrrhachium in July of 48 BC, Caesar moved swiftly into Thessaly, incorporating the towns of the region under his control. His exhausted and poorly supplied army was able to secure new sources of food and essentially become re-energized for the continuing campaign. After Dyrrhachium, Pompey and the Senators squabbled over the next course of action, and they pressed Pompey hard to finish Caesar as quickly as possible. Pompey preferred a course of action similar to that of Fabius against Hannibal; keep Caesar from becoming secure in a single location, constantly threatening his supply and resisting major battles whenever possible. With their success at Dyrrhachium, however, Pompey's initially fearful legions were now filled with confidence against the vaunted conqueror of Gaul. This exuberance, coupled with Senate pressure, and Pompey's own lack of decisiveness was to prove a fatal mix.
Meanwhile, as the two armies marched and jockeyed for position, Pompey was joined by Metellus Scipio's legions from the east. Domitius Calvinus, who was detached by Caesar earlier in the year two stop Scipio, returned to Caesar as well, putting both armies at full strength. On the plains of Pharsalus, just north of the Enipeus River, the two armies moved into position opposing one another. Pompey vastly outnumbered Caesar with some 45,000 infantry and 7,000 cavalry to Caesar's 22,000 and 1,000 respectively. It's important to note, however, that a considerable part of Pompey's forces were allied auxilia sent from his eastern clients, and not fully trained Roman legions. Pompey arranged his forces and offered battle on a hill called Mount Dogantzes, and Caesar was certainly elated. This is exactly what he needed, the opportunity to face the enemy on open ground in a battle where his men were well supplied and in good order.
After several days skirmishing and jockeying for position, Caesar so effectively taunted Pompey that he eventually forced him into taking up position on level ground. On August 9, 48 BC, the pivotal battle for control of the Roman world was set to begin. Finding his army in the best of circumstance, Caesar inspired his men and prepared his lines: "Our march at present, and set our thoughts on battle, which has been our constant wish; let us then meet the foe with resolute souls. We shall not hereafter easily find such an opportunity." Pompey's army was arranged with his right wing, Cilician legionaries and Spanish auxilia, protected by the river under the command of Cornelius Lentulus. In the center, Syrian and African troops were led by Scipio. On his left is where Pompey hedged all his bets and hoped for victory by shear force of numbers. The infantry was commanded by Lucius Domitius Ahenobarbus, and on the flank, Pompey placed his entire cavalry, archers and slingers under Caesar's former legate, Titus Labienus. Outnumbering Caesar's cavalry 7:1, Pompey and his army were confidant that they could easily outflank Caesar's right and win the war swiftly with minimal bloodshed.
Caesar, however, saw a grand opportunity to counter Pompey's seemingly all or nothing plan. Marcus Antonius was placed on the left, Calvinus in the center, and Publius Sulla on the right. Caesar's infantry was thinned out to match the length of Pompey's numerically superior lines, decreasing the depth of his lines, but protecting the flanks. Caesar's plan only required his infantry to hold firm, not be the force that broke through. His much smaller cavalry was placed on the right to counter Pompey's cavalry, but the weakness in numbers was a serious threat. Caesar, however, also realized that this would obviously be what seemed to be the vulnerable target and would be irresistible. He then further reduced his main infantry lines, drawing 3,000 of his best men from among the various cohorts. These men he positioned somewhat concealed behind his cavalry and right flank infantry. This unit was to be the key to the battle, if they were able to use the element of surprise to counter Labienus' cavalry, it would be Pompey's wing which would be routed and flanked, not Caesar's.
With both armies set, it was Caesar and not Pompey who ordered the initial advance. Pompey hoped the long charge would tire Caesar's army, but the veterans understood the danger and stopped when they noticed that the enemy wasn't coming out to meet them. The battle slowly developed as an infantry skirmish in the center until Pompey finally unleashed Labienus and the cavalry. Pompey's horsemen hit Caesar's Germanic and Gallic cavalry hard, buckling their resistance. Pompey ordered his archers and slingers to fill in behind the cavalry to push the assault and provide a heavy blanket of covering fire. Just as Caesar's cavalry was beginning to retreat, and Labienus was starting to turn the right flank, Caesar ordered his reserve infantry to launch their surprise assault. Using their pila much like medieval pikes, Caesar's 3,000 infantry attacked the 7,000 Pompeian cavalry with ferocity, targeting the riders exposed faces. The effect was devastating, and Labienus was overwhelmed. The cavalry routed and turned towards its own lines, not only leaving their own vulnerable archer units completely exposed but likely trampling many as they went.
Caesar now wheeled around on Pompey's exposed left flank. Cutting the archers and slingers to pieces, they hit the Pompeian lines hard, crumbling the flank. Pompey, still with a vast numerical superiority seems to have panicked and failed to engage his right wing to stem Caesar's momentum. Instead, he simply quit the battle rather than attempt to rally, or salvage what he could. Pompey retreated and retired to his fortified camp while his army was routed, waiting for the imminent arrival of the victor. Caesar, meanwhile, pressed his advantage. He encouraged the remaining Pompeian legionaries to withdraw without more bloodshed while instilling in his men not to attack their fellow Romans provided they offered no resistance. Instead he smashed what remained of Pompey's auxiliary allies, leaving a devastating wake as he approached Pompey's camp.
At this point, Pompey seems to have regained his senses, but still he didn't act with the honor of a noble Roman. Rather than fall upon his own sword in the Roman tradition, Pompey fled the camp, leaving his army to the enemy. Caesar entered the camp to find that the command tent had been arranged in such a manner to receive an elegant feast and laurels of victory, clearly indicating the supreme confidence of his opponents. Conveniently taking advantage of this Pompey's gift, Caesar also captured his rival's personal papers and effects. In a shrewd political move, yet unfortunate event for history, Caesar burned Pompey's papers supposedly without reading them, in order to bring closure to the matter and restore a sense of unity in Rome.
As the battle closed, Caesar reviewed the field and was likely shaken by the effects of civil war. He claimed that 15,000 enemy soldiers were killed, including 6,000 Romans, while losing only 200 of his own men, though both numbers are likely either over or under exaggerated. Still, the sight of the field apparently had a profound effect on the new master of the Roman world. In surveying the carnage, Caesar supposedly said, "They would have it so, I, Gaius Caesar, after so much success, would be condemned had I dismissed my army."
The following day, the remaining Pompeian forces surrendered to Caesar, and the major part of the war was essentially over. Though some Senators fled to Africa or other Republican strongholds, many of Caesar's most vocal enemies were killed in the campaign. Pompey himself fled to Egypt, where his own horrible fate awaited him. Respected as the conqueror of the east, Pompey certainly felt comfortable heading into Egypt. While waiting off-shore to receive word from the boy-king, Ptolemy, Pompey was betrayed and assassinated. Stabbed in the back and decapitated, his body was burned on the shore and his head was brought to the king in order to present as a gift to Caesar. On July 24, 48 BC, Gnaeus Pompeius Magnus was dead, just short of 58 years old. Despite Pompey's prestige in the east, the legend of Caesar must've been incredible. The man had conquered Gaul, crossed the Rhine, crossed into the farthest reaches of the known world in Britannia, and now utterly destroyed the Great Pompey with a far inferior force. When Caesar arrived in pursuit of Pompey, to certainly, by all accounts, grant him a pardon and welcome him back to Rome, Ptolemy presented Caesar with Pompey's head and his signet ring. Caesar, despite realizing Pompey's death made him the master of Rome, was overcome with grief. Turning away from the slave who presented Pompey's head, Caesar burst into tears at the sight of his rival, former friend, and son-in-law.