Later, in 1578, along with his father, he participated in the royal crusade to Morocco, and he fell prisoner to Ahmad al-Mansur's armies during the Battle of Ksar El Kebir.
He returned to Europe through San Lucar, in southern Spain, where he was received with all honours by the Duke of Medina Sidonia, Alonso de Guzman.
As he testified of the King's death during the battle, there and for the first time he supported Dom António, Prior of Crato as the natural heir of the Portuguese throne, refuting the Spanish Duke's position for Philip II of Spain's claims.
He was appointed by the new King as his Constable, and they participated together in the Battle of Alcântara (1580), defending Lisbon from the Spanish armies, led by the Duke of Alba.
In 1582 he sailed, in the fleet led by the Italian adventurer Filippo Strozzi, to the Azores, the sole Portuguese territory that still recognised Anthony I as King of Portugal.