Sebastian, 1554–78, king of Portugal (1557–78), grandson and successor of John III. He was under the regency first of his grandmother (until 1562) and then of his uncle Henry (a cardinal and later king) until declared of age in 1568. Weak and sickly, Sebastian was imbued by his Jesuit training with fanatic religious fervor, which he combined with a tremendous admiration for the military. Thus he viewed himself as a Christian knight and became determined to win glory by fighting the Muslim infidels in Africa. An appeal for help from a pretender to the Moroccan throne gave Sebastian his opportunity. Having secured (1576) a promise of aid from Philip II of Spain (his uncle), he spent vast sums of money in preparation for an expedition against Morocco. In 1578, Sebastian set out with a large force of foreign mercenaries (but no Spanish) and landed in Morocco. His headstrong desire to command and his lack of experience led to defeat in the Battle of the Three Kings at Ksar el Kebir. His army was wiped out, and he himself was killed. However, it was not known at the time that he was dead, and rumor had it that he had been captured. The legend that he would return persisted, and Sebastianism (a messianic religious belief) lasted well into the 19th cent. His uncle Henry succeeded him and was the last of the Aviz kings. The crown then went to Philip II, and Spanish control of Portugal began.

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