He was educated almost entirely by Jesuits, by his guardian and tutor Aleixo de Meneses and by Catherine of Austria, sister of Charles V and wife of King John III.

Sebastian felt driven to revive lost glories by intervening in North Africa, influenced by the events such as the defense of Mazagan in 1562 from a Moroccan siege.

Up to that time Portuguese military action in Africa had been confined to small expeditions and raids; Portugal had built its vast maritime empire from Brazil to the East Indies by a combination of trade, sea exploration, and technological superiority, with Christian conversion of subject peoples being one, but by no means the only, end in view.

Despite the admonitions of his mother and his uncle Philip II of Spain (who had become very cautious after the Battle of Djerba), Sebastian was determined to wage a military campaign, and he used much of Portugal's imperial wealth to equip a large fleet and gather an army which included soldiers of several nationalities: 2,000 volunteers from Spain (Castile), 3,000 mercenaries from Flanders and Germany, and 600 Italians initially recruited to aid in an invasion of Ireland under the leadership of the English adventurer, Thomas Stukley.

Their rise to power began as a resistance to the Portuguese presence in Agadir and by the 1550s they controlled most of present-day Morocco and had supplanted the earlier Wattasid dynasty.

[12] Upon the death of Sultan Abdallah al-Ghalib in 1574 his son Muhammad II al-Mutawakkil inherited the throne, but two years later he was overthrown by his uncle Abd al-Malik.

[12][13][14]: 188  After this, the Saadi army, whose bulk was recruited from guich troops ("military tribes" mobilized to serve as regular levies[15]), combined mounted arquebusiers, infantry armed with rifles, large numbers of light cavalry, and a detachment of artillery; most of which were newly trained in Ottoman tactics.

[18] In an encyclopedic entry about King Sebastian, historian Allen Fromherz indicates the presence of Ottoman forces at the battle, including Janissaries.

The battle ended after nearly four hours of heavy fighting and resulted in the total defeat of the Portuguese and Abu Abdallah's army with 8,000 dead, including the slaughter of almost the whole of Portugal's nobility.

[23] The Sultan Abd Al-Malik died during the battle from natural causes (the effort of riding was too much for him), but the news was concealed from his troops until total victory had been secured.

Abd Al-Malik was succeeded as Sultan by his brother Ahmad al-Mansur, also known as Ahmed Addahbi, who conquered Timbuktu, Gao, and Jenne after defeating the Songhai Empire.

The Moroccan army which invaded Songhai in 1590–91 was made up mostly of European captives, including a number of Portuguese taken prisoner at the battle of Alcácer Quibir.

Later, at the beginning of his reign, Philip II ordered that the mutilated remains said to be Sebastian's (and so recognized after the battle by some of his close companions),[citation needed] and still in North Africa, be returned to Portugal, where they were buried at the Jerónimos Monastery, in Lisbon.

Partly in reaction to the national trauma of this disastrous defeat, a cult of 'Sebastianism' which portrayed the lost monarch in terms similar to King Arthur arose.