Spain claimed the islands in 1592, and the European powers recognized its sovereignty over the islands in 1874. They had been part of the Spanish East Indies formally since 1528. Later, Spain sold some of the islands to the German Empire in 1885, and they became part of German New Guinea that year, run by the trading companies doing business in the islands, particularly the Jaluit Company. In World War I, the Empire of Japan occupied the Marshall Islands, which, in 1920, the League of Nations combined with other former German territories to form the South Seas Mandate. During World War II, the United States took control of the islands in the Gilbert and Marshall Islands campaign in 1944. Nuclear testing began on Bikini Atoll in 1946 and concluded in 1958.