The battle of Pharsalus (9 August 48 BC) was the decisive battle of the Great Roman Civil War, and saw Caesar defeat Pompey and the Senate’s main army. Although the war continued for another three years, Pharsalus ended any realistic chance that Caesar could be defeated, and the war would have ended soon if Caesar hadn’t become entangled in Egyptian affairs.
At the start of the civil war Pompey decided that he couldn’t defend Rome against Caesar’s rapidly advancing veterans, and decided to retreat to the Balkans. After failing to catch him at Brundisium, Caesar decided to deal with his army in Spain first, eventually defeating it at Ilerda. Only then did he turn back to deal with Pompey’s increasingly powerful army in the Balkans. Although Pompey commanded a powerful fleet, he was unable to stop Caesar crossing to the Balkans. A long stalemate then developed at Dyrrhachium (on the coast of modern Albania). This ended with a rare battlefield defeat for Caesar (battle of Dyrrhachium, 20 May 48 BC), after which Caesar decided to end the siege and adopt a new policy.
Caesar’s new plan was to advance east into Thessaly, where his legate Domitus Calvinus was being threatened by a Senatorial army under Metellus Scipio, newly arrived from Syria. Pompey was left with the choice between taking the war back to Italy or pursing Caesar, and chose the later option. For a few days Pompey attempted to catch Caesar’s retreating army, but soon gave up and followed at a more leisurely pace.
The two armies ended up camped close to Pharsalus in Thessaly. Caesar was in a difficult position - outnumbered, short of supplies and surrounded by hostile locals. Pompey realised this, and would have preferred to besiege Caesar and starve him out. However Pompey wasn’t entirely in command of his own army, which was accompanied by a crowd of senators who saw him as ‘their’ commander and complained whenever he delayed. Eventually the pressure got to Pompey, and he agreed to risk a battle.
This decision came just in time for Caesar. On the morning of the battle Pompey drew up his army at the foot of the heights he was camped on, and offered battle. Caesar decided that it wasn’t worth risking an attack on this strong position, and decided to break camp and move off, in the hope that Pompey would make a mistake in the pursuit. Just as Caesar’s men were preparing to move off, he realised that Pompey had moved further out from the mountains and there was now a chance for a battle on more equal terms.
Caesar reported that Pompey had 110 cohorts, or 45,000 men in his army, along with two cohorts of volunteers. He also had 7,000 cavalry. His right flank was protected by the Enipeus River. He posted a Cilician legion and his surviving Spanish troops on his right. Metellus Scipio commanded in the centre with the army he had brought from Syria. Pompey himself commanded on the left, where he posted two legions that Caesar had given to him before the outbreak of the civil war, when the Romans were planning to fight the Parthians in Syria. The cavalry, slingers and archers were all placed on the left.
Pompey’s plan was to use his superior cavalry to outflank and defeat Caesar’s right wing, and from there role up the rest of the army.
Caesar had eighty under-strength cohorts, a total of 22,000 men. He only had 1,000 cavalry. He placed the 9th and 10th legions on the left, commanded by Mark Antony. Domitius Calvinus was in the centre and P. Sulla on the right, as was Caesar, who placed himself at the head of the 10th legion, facing Pompey. Pompey’s disposition made it clear that his plan was to attack around Caesar’s right flank, and so he took six cohorts from his rear line and placed them on the right, with orders to stop Pompey’s cavalry.
Pompey ordered his men to stand their ground and wait for Caesar’s attack to reach them, instead of taking the normal step of a counter-charge. His theory was that this would leave his men fresher than Caesar’s, and reduce the power of his javelins, but Caesar believed that it reduced the enthusiasm of Pompey’s men, who had to passively stand and wait to be attacked. In the event this plan had little impact, as Caesar’s men simply paused for a rest after marching halfway across the gap between the two armies.
The battle began with a clash between the two lines of infantry. Once Caesar’s men were committed, Pompey ordered his cavalry to attack. They were able to push back Caesar’s smaller cavalry force as planned, but were then attacked by Caesar’s six reserve cohorts. Pompey’s cavalry was caught out of formation, defeated and forced to flee from the battle. The archers and slingers were left without protection, and were also defeated. The six cohorts then outflanked Pompey’s left flank and attacked it from the rear. At this point Caesar ordered his third line to join the battle. Pompey’s left wing was now close to defeat. According to Caesar Pompey himself retired to his camp, and took shelter in his tent. Pompey’s infantry now retreated into their camp, with Caesar’s men close behind. Caesar convinced his men to attack the enemy camp before they had time to restore order. The camp was defended by the cohorts that had been left behind for that purpose and Pompey’s Thracian allies, but the defeated troops from the main army didn’t contribute much. Soon Caesar’s men were able to break into the camp, and the survivors of Pompey’s army fled into the mountains.
Caesar claimed to have only lost 200 men during the battle, amongst then 30 centurions. In contrast he gave casualty figures of 15,000 for Pompey, along with 24,000 prisoners. Amongst the dead was Domitius Ahenobarbus, but many of the surviving Senators were forgiven by Caesar. Most famous of these was Marcus Brutus, later one of the leaders of Caesar’s assassins. Cicero, who had not been present at the battle, also decided to seek Caesar’s forgiveness. Cato, who had also not been with the army, escaped to Africa, where he joined up with Metellus Scipio. Between them they raised the last significant Republican army, eventually forcing Caesar to move against them. Eventually he caught and defeated them at Thapsus (47 BC).
After a day or two most of the survivors surrendered to Caesar, and were treated with his normal mercy. Some of the surviving noblemen fled, and either escaped into exile or joined the remaining Republics back on the west coast.
In the aftermath of the battle Pompey fled to the coast, where he found a friendly ship. He fled to Lesbos, where he joined with his wife. From there he moved to Egypt, where he expected to receive aid from his client Ptolemy XIII. Instead he was murdered on the beach. Caesar was close behind, and reached Alexandria three days later. Caesar was greatly angered by the Egyptian treachery, and soon got dragged into Egyptian politics, siding with Ptolemy’s sister Cleopatra VII. Caesar ended up being besieged in Alexandria for sixth months, giving his enemies one last chance to unite against him, but without success.