Karafuto Prefecture
During the Japanese colonial period of 1910–1945, Koreans were often recruited or forced into indentured servitude to work in mainland Japan, Karafuto Prefecture (Sakhalin) and Manchukuo, especially in the 1930s and early 1940s; the ones who chose to remain in Japan at the end of the war became known as Zainichi Koreans, while the roughly 40 thousand who were trapped in Karafuto after the Soviet invasion are typically referred to as Sakhalin Koreans. According to the statistics at Immigration Bureau of Japan, there were 901,284 Koreans resident in Japan , of whom 515,570 were permanent residents and another 284,840 were naturalized citizens.