Between April and June 1563 the Regency of Algiers launched a major military campaign to retake the Spanish military-bases of Oran and Mers el Kébir on the North African coast, occupied by Spain since 1505.

The Kingdom of Algiers, the Principalities of Kabyle (Kuku and Beni Abbes), and other vassal tribes combined forces as one army under Hasan Pasha, son of Hayreddin Barbarossa, and Jafar Catania.

With the defeat of the Knights of Malta and the conquest of Tripoli by the Ottoman admiral Turgut Reis in 1551, and the fall of Bougie to the governor of Algiers, Salih Reis, in 1555, Oran and Mers El Kébir were, along with the island of La Goulette, the only remaining possessions of Christendom in Barbary resisting Ottoman and Moorish piracy operating along the coasts of Naples, Sicily, and the Levant.

[9] The siege began on April 3, 1563, when Ottoman troops massively attacked the tower of Todos los Santos, defended by 200 Spanish soldiers.

Ottoman's main objective was to capture Mers El Kébir, as Hassan was warned by several renegades that the Spanish planned to abandon Oran to concentrate on the defense of the other town.

[10] However, despite the stubbornness of the defense, the reinforcements sent from Mers el Kébir were not enough to continue fighting, and on May 8, under cover of darkness, the Spanish survivors retreated to the town.

Hassan sent ahead 12,000 Arabs to break the resistance of the Spanish harquebusiers and facilitate the assault by two columns of regular troops which would attack secondly.

[14] King Philip II, informed about the development of the siege, decided to reward Martín de Córdoba and Francisco Vivero, commanding officer of Fort San Miguel, for keeping these two crucial strongholds in Spanish hands.

[15] King Philip II ordered Vespasian Gonzaga Colonna, Prince of Sabbioneta and Duke of Trayecto, to make a comprehensive report about the situation of both towns.