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king of Aragon from 1285 to 1291, son of Peter III. A weak king, he was involved in an unsuccessful constitutional struggle with the Aragonese nobles. In 1287 he was compelled to grant the so-called "Privilegio de la Union," which handed over a number of important royal prerogatives to baronial control. At Alfonso's death the crown passed to his brother James II, who had been king of Sicily (as James I) since 1285.