After the capture of Makin and Tarawa in the Gilbert Islands, the next step in the United States Navy's campaign in the central Pacific was the Marshall Island chain. These islands had been Imperial German colonies, after their purchase from Spain in 1899. At the end of World War I they were assigned to Japan in the post-war settlement as the "Eastern Mandates". The islands then became a mystery because the Japanese closed them to the outside world. It was presumed the Japanese had built illegal fortifications throughout the islands, but the precise extent of such fortifications was unknown. Japan regarded them as part of the "outer ring" of their territory.