Sultan Abu al-Hasan Ali of the Marinids had assembled an invasion force that crossed the Strait of Gibraltar in the summer of 1340 -- 60 war galleys, 250 other ships, and an army that some chronicles estimated at 60,000 men. The crossing represented the Marinids' final attempt to establish a permanent power base in Iberia. Earlier that year, in April, the Marinid fleet had destroyed the Castilian navy at the Battle of Getares, killing Admiral Alfonso Jofre de Tenorio and capturing 28 galleys. With naval supremacy secured, Abu Hasan landed at Gibraltar on August 14 and, joined by Yusuf I of Granada, laid siege to Tarifa on September 22. Then he made a critical error: believing the Castilians would need months to rebuild their fleet, the sultan decommissioned most of his galleys to save costs, leaving only twelve at Algeciras.

Alfonso XI moved fast. He secured Portuguese naval support from his father-in-law, King Afonso IV, whose admiral Manuel Pessanha brought a fleet that was reinforced with fifteen Genoese galleys and twenty-seven ships rapidly completed at Seville. By October, the Christian fleets controlled the strait, severing the supply line between Morocco and the invasion army. Abu Hasan's position became desperate. On October 10, a storm wrecked twelve Castilian galleys, and the sultan launched an all-out assault on Tarifa that was barely repulsed with heavy losses on both sides. Alfonso departed Seville on October 15 with his relief army, joining the Portuguese king the next day. By October 26, their combined force of roughly 20,000 crossed into contested territory. The Marinids lifted the siege and took positions on hills overlooking the Salado valley.

The battle began at nine in the morning. Alfonso's vanguard, led by the Lara brothers, advanced toward the Salado River but met fierce resistance and could not cross. The king committed 1,500 additional knights, and the crossing succeeded. On the left, 3,000 Castilian knights reinforced the Portuguese contingent against Yusuf I's 7,000 Granadan cavalry. In the center, Juan Nunez de Lara and the Knights of Santiago broke through with 3,000 horsemen and rode uphill toward the sultan's camp, while the troops hidden in Tarifa attacked from the rear. Alfonso himself nearly entered hand-to-hand combat before the Archbishop of Toledo, Gil Alvarez Carrillo de Albornoz, seized his horse's reins and stopped him. By noon it was over. Abu Hasan's army broke and fled toward Algeciras. The pursuit was relentless, ending six kilometers away at the Guadameci River.

The human cost was terrible. Many of the sultan's household were killed in the rout, including his wife Fatimah, daughter of the Sultan of Tunis, and Aysa, daughter of the nobleman Abu Yahya ibn Yaqub. Other relatives were taken captive. Abu Hasan himself reached Algeciras, crossed to Gibraltar, and that same night fled to Ceuta in a galley. The Marinids retreated to Africa permanently. No Muslim army would ever again invade the Iberian Peninsula. Control of the Strait of Gibraltar passed definitively to the Christians -- the Castilians and their Genoese allies. The war with Granada continued another decade, during which Alfonso retook Algeciras after a two-year siege in 1344, but died of the Black Death at his camp in 1350. Gibraltar itself would not fall to Castile until 1462, more than a century later.