Duarte de Menezes reputation and experience prompted Peter to appoint him to sensitive military posts. Duarte led Portuguese incursions in 1441 and 1444, to assist the Crown of Castile against the rebellious Infantes of Aragon. At the request of John II of Castile, Duarte de Menezes was subsequently posted to serve on the frontier of the Emirate of Granada, but stayed only a few months. In August 1444, regent Peter of Coimbra appointed Duarte de Menezes to the high office of "alferes-mor" (standard-bearer) of the realm. Duarte accompanied constable Peter of Portugal (regent Peter's son) in another interventionist incursion into Castile in July 1445. The relationship between Peter and Duarte de Menezes ended abruptly when King Afonso V of Portugal reached majority in 1448 and dismissed the regent Peter. Afonso V immediately dispatched Duarte de Menezes as military governor of Pombal, a critical fortress on the borderlands of Coimbra, to keep a check on the dismissed regent's movements. Duarte de Menezes fought for the king against Peter at the Battle of Alfarrobeira in May 1449. In reward for his loyalty, Duarte de Menezes' royal pensions were expanded, and his crown-granted titles, including "alferes-mor", confirmed inheritable in his family. In October 1458, King Afonso V of Portugal launched a new operation against Morocco, the first since the disaster at Tangier in 1437. The Marinid sultan Abd al-Haqq II, then laying siege to Tlemcen, heard the news of the massive Portuguese expeditionary force and presuming it was another attempt at Tangier, sent the bulk of his forces to that city. Instead, the Portuguese fleet swooped down on Ksar es-Seghir (Port: "Alcácer-Ceguer"), seizing the city in a two-day assault on October 23–24, 1458. Afonso V appointed Duarte de Menezes as the first captain and governor of Alcácer-Ceguer. Duarte de Menezes first order of business was to prepare the defenses of the citadel for the inevitable reaction by the Marinids. The Moroccan army, led by Abd al-Haqq II, laid siege to the Alcácer-Ceguer in November 1458.