Operation Blacklist Forty was the codename for the United States occupation of Korea between 1945 and 1948. Following the end of World War II, U.S. forces landed within the present-day South Korea to accept the surrender of the Japanese, and help create an independent and unified Korean government with the help of the Soviet Union, which occupied the present-day North Korea. However, when this effort proved unsuccessful, the United States and the Soviet Union both established their own friendly governments, resulting in the current division of the Korean Peninsula.

The division of Korea began on August 15, 1945 when the official announcement of the surrender of Japan was released, thus ending the Pacific Theater of World War II. During the war, the Allied leaders had already been considering the question of Korea's future following Japan's eventual surrender in the war. The leaders reached an understanding that Korea would be liberated from Japan but would be placed under an international trusteeship until the Koreans would be deemed ready for self-rule. In the last days of the war, the United States proposed dividing the Korean peninsula into two occupation zones with the 38th parallel as the dividing line. The Soviets accepted their proposal and agreed to divide Korea.