The harbor of Mostaganem had been captured by the Spanish from the Muslims in 1506. The harbor became part of numerous Spanish possession on the North African coast that had been captured since 1496: Melilla (1496), Mers-el-Kebir (1505), Oran (1509), Bougie (1510), Tripoli (1510), Algiers, Cherchell, Dellys, and Tenes.

Mostaganem had in 1516 been seized by Hayreddin Barbarossa from his base atAlgiers, who then strengthened its defenses; the city then became a rival of Oran.

Barbarossa had from 1519 placed himself under the protection of the Ottomans, thereby transforming Mostaganem into an Ottoman dominion.

Previous Spanish expeditions had taken place in 1543 and 1547, which failed as the Spanish forces were repulsed and then pursued in retreat by Turkish and tribal forces.

The expedition of 1558 to retake Mostaganem from the Ottomans follows a string of Ottoman successes in the Mediterranean, especially with the Siege of Tripoli in 1551, and the evacuation of Al-Mahdiyeh by the Spaniards.

Concurrently, the corsairs of Barbary are operating from their base in Algiers.

The Ottomans had in 1557 demanded the submission of Morocco to the Ottoman Empire, but had murdered the Moroccan ruler Mohammed ash-Sheikh, who was collaborating with Spain, when he refused.

The Ottomans under Hasan Pasha, the son of Barbarossa and ruler of Algiers, had in March 1558 invaded the Moroccan territory toward Fez, but were stopped at the indecisive Battle of Wadi al-Laban just north of the city.

The Ottomans retreat when they learn about Spanish preparations for an offensive in Oran.

An elite contingent, numbering sixty-five hundred, had been dispatched from Málaga by Philip II of Spain, and concentrated at the Spanish base of Oran to join local Spanish troops and the Moroccan Goum troops of the Caïd of Tlemcen, Abdallah al-Ghalib.

They had departed from Oran on August 22 or 26, 1558.

The Spanish expedition attacks Mostaganem but is repulsed.

With supplies running low, they learn that Hasan Pasha is arriving from Algiers with a relief force of about six thousand troops.

This leads to a panicked retreat by the Spanish troops.

In the end, around twelve thousand Spanish soldiers are taken prisoner.

Count Alcaudete, Governor of Oran, dies in the expedition; his son Don Martín de Córdoba, himself future Governor of Oran, is also captured in the disaster and will be imprisoned as a Christian slave in Algiers under the beylerbey Hasan Pasha, until he is exchanged for the huge ransom of twenty-three thossand escudos.

The failure of the expedition of Mostaganem ends attempts at a grand alliance between Spain and Morocco against the common Ottoman enemy.