Achillas commanded Ptolemy XIII’s army against Julius Caesar in Alexandria. He was a leading guardian of the young king and held command of the Egyptian forces during the Alexandrian War.
After Caesar arrived in 48 BC, Ptolemy’s regent, Pothinus, ordered Achillas to bring a large army from Pelusium to Alexandria. Ancient accounts give the force as roughly 20,000 infantry and 2,000 cavalry. Achillas rejected Caesar’s attempts to negotiate and began the siege of the royal quarter.
Achillas did not command the Egyptian army throughout the entire conflict. Arsinoe IV, Ptolemy XIII’s sister, escaped Caesar’s control, joined the besiegers, and arranged Achillas’s execution after a power struggle. The eunuch Ganymedes then took command. This later change is why Achillas can sometimes be confused with the other Egyptian commanders in the siege.
The Egyptian forces were eventually defeated after Caesar received reinforcements and won the Battle of the Nile. Ptolemy XIII died while fleeing, and Caesar restored Cleopatra to power alongside their younger brother, Ptolemy XIV.