At the onset of the Spanish–American War, de los Reyes was employed as Counselor of the Ministry of the Colonies ("Consejero del Ministerio de Ultramar"), which he held until 1901. In this capacity, de los Reyes helped rally Filipino support against the Americans, thinking that this would create conditions favorable to the Philippines. He believed that once the Americans were repelled, they would be granted autonomy, and should Spain renege, then the already armed Filipinos could take matters to their own hands. He had received assurances from the governor-general Basilio Augustin regarding autonomy, and together with other Filipinos in Spain, offered to return to the Philippines to organize militias to fight the Americans.