After the Battle of Pharsalus, Pompey abandoned his defeated army and fled with his advisors overseas to Mytilene and thence to Cilicia where he held a council of war.

[3] Upon his arrival in Egypt, he was murdered by Achillas and Lucius Septimius, former soldiers in his army, under the orders of the eunuch Pothinus and Theodotus of Chios,[4][5][6] advisors of the King Ptolemy who believed Caesar would be pleased by the removal of his adversary.

[7] Caesar landed in Alexandria three days after Pompey's death with some three thousand men and eight hundred Germanic auxiliary horse, arrogantly occupying parts of the Alexandrian royal quarter.

[10][11] After the payment demand, Pothinus sent secret orders summoning Achillas and an army of some twenty thousand men to Alexandria, where they besieged and then launched an all-out attack on the Royal Quarter.

[13] Meanwhile, Arsinoe IV, the younger sister of Ptolemy, escaped from Caesar and joined the Egyptian army, which proclaimed her queen.

[13] She arranged successfully with her eunuch tutor Ganymede the murder of Achillas and then assumed command of the army, renewing the siege.

Caesar unwilling to give up his naval superiority drew up his own fleet, 19 warships and 15 smaller vessels, in two lines just north of the coast of Pharos Island.

Caesar's captains decided to take the initiative themselves by landing archers and slingers on the bridge to fend off the enemy ships.

In the panic, Caesar's craft was swamped by soldiers, forcing him to remove his armour and then swim to shore, holding his left hand above water to save some important documents.

[14] Ptolemy XIII, feigning fear of being sent away, was released; he promptly joined his sister and urged his soldiers to continue the attack on Caesar.

The situation began to turn in Caesar's favour when news reached him in March 47 BC of a relief force arriving overland from Syria under Mithridates of Pergamum at the head of an allied army with a detachment of three thousand Jews contributed by High Priest Hyrcanus II and led by Antipater the Idumaean.