The correct answer is a- "They sent in troops to occupy North Korea at the end of WW2, when Japan was defeated and had to withdraw from the Korean peninsula". The Korean peninsula was a Japanese colony. Japan lost control of Korea when it surrendered to the Allied Powers - Great Britain, the Soviet Union and the United States -- in 1945. By 1949, the United States and the Soviet Union had removed their forces from Korea. In an attempt to reunify the peninsula under communist rule, on June 25, 1950, with Soviet approval, North Korea launched an assault on South Korea.