The Battle of Pharsalus was the decisive battle of Caesar's Civil War fought on 9 August 48 BCE near Pharsalus in central Greece. Julius Caesar and his allies formed up opposite the army of the Roman Republic under the command of Pompey. Pompey had the backing of a majority of Roman senators and his army significantly outnumbered the veteran Caesarian legions.

Pressured by his officers, Pompey reluctantly engaged in battle and suffered an overwhelming defeat. Pompey, despairing of the defeat, fled with his advisors overseas to Mytilene and thence to Cilicia where he held a council of war; at the same time, Cato and supporters at Dyrrachium attempted first to hand over command to Marcus Tullius Cicero, who refused, deciding instead to return to Italy. They then regrouped at Corcyra and went thence to Libya. Others, including Marcus Junius Brutus sought Caesar's pardon, travelling over marshlands to Larissa where he was then welcomed graciously by Caesar in his camp. Pompey's council of war decided to flee to Egypt, which had in the previous year supplied him with military aid.

In the aftermath of the battle, Caesar captured Pompey's camp and burned Pompey's correspondence. He then announced that he would forgive all who asked for mercy. Pompeian naval forces in the Adriatic and Italy mostly withdrew or surrendered.