The Battle of Pharsalus was the decisive battle of Caesar's Civil War fought on 9 August 48 BC 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, ultimately fleeing the camp and his men, disguised as an ordinary citizen. Eventually making his way to Egypt, he was assassinated upon his arrival at the order of Ptolemy XIII.

Caesar's Civil War (49–45 BC) was one of the last politico-military conflicts of the Roman Republic before its reorganization into the Roman Empire. It began as a series of political and military confrontations between Gaius Julius Caesar and Gnaeus Pompeius Magnus.

Before the war, Caesar had led an invasion of Gaul for almost ten years. A build-up of tensions starting in late 49 BC, with both Caesar and Pompey refusing to back down led, however, to the outbreak of civil war. Eventually, Pompey and his allies induced the Senate to demand Caesar give up his provinces and armies. Caesar refused and instead marched on Rome.

The war was a four-year-long politico-military struggle, fought in Italy, Illyria, Greece, Egypt, Africa, and Hispania. Pompey defeated Caesar in 48 BC at the Battle of Dyrrhachium, but was himself defeated decisively at the Battle of Pharsalus. Many former Pompeians, including Marcus Junius Brutus and Cicero, surrendered after the battle, while others, e.g. Cato the Younger and Metellus Scipio fought on. Pompey fled to Egypt and was killed upon arrival. Scipio was defeated in 46 BC at the Battle of Thapsus in North Africa. He and Cato committed suicide shortly after the battle. The following year, Caesar defeated the last of the Pompeians under his former lieutenant Labienus in the Battle of Munda and became dictator perpetuo (Dictator in perpetuity or Dictator for life) of Rome.