In 1914, during World War I, the island was captured by the Empire of Japan. Japan was awarded formal control of the island in 1919 by the League of Nations as a part of its mandated territory of the South Seas Mandate. Militarily and economically, Saipan was one of the most important islands in the mandate and became the center of subsequent Japanese settlement. Immigration began in the 1920s by ethnic Japanese, Koreans, Taiwanese and Okinawans, who developed large-scale sugar plantations. The South Seas Development Company built sugar refineries and, under Japanese rule, extensive infrastructure development occurred, including the construction of port facilities, waterworks, power stations, paved roads and schools, along with entertainment facilities and Shinto shrines. By October 1943, Saipan had a civilian population of 29,348 Japanese settlers and 3,926 Chamorro and Carolinian Islanders.