Almost immediately after coming of age, Sebastian began plans for a great crusade against the Moroccans. The Portuguese crossed into Morocco in 1578 and, against the advice of his commanders, Sebastian marched deep inland. At the Ksar El Kebir (Field of the Three Kings) the Portuguese, along with crusaders and several mercenaries from different parts of christiandom, were routed by deposed Moroccan Sultan Abu Abdallah Mohammed II, and after the disastrous defeat against the Saadians of Morocco, Sebastian was almost certainly killed in battle or subsequently executed. But for the Portuguese people, he had just disappeared and would return home one day, to such an extent that, in 1640, King John IV of Portugal had to swear to yield his throne to Sebastian, in case Sebastian (who would have been 86 years old) were to return.