'Blacklist Forty' was the US occupation of southern Korea (4 September 1945/48).
After the end of World War II, US forces landed in the area of Korea to the south of 38° N to take the surrender of the Japanese forces which had occupied Korea since 1910, and to help in the establishment of an independent and unified Korean government with the help of the USSR, which had occupied the area of Korea to the north of 38° N at the end of 'Avgust Buri'. When this effort proved unsuccessful, however, the USA and USSR each established a Korean government within its sphere of influence, and thereby divided the Korean peninsula.
The partition of Korea into separate occupation zones had been proposed in August 1945 by the USA and USSR after the latter’s entry in the war against Japan. The 38th parallel was chosen to separate the two occupation zones on 10 August by two US officers, Dean Rusk and Charles Bonesteel, working on short notice and with little information on Korea. The two men’s superiors endorsed the partition line and the proposal was accepted by the USSR. The Americans hoped to establish a representative government supportive of US policy in the region, and the Soviets hoped to establish another communist nation friendly to their interests.
The US occupation forces comprised 45,000 men of Major General John R. Hodge’s XXIV Corps, and the first elements of this formation to reach Korea were a small advanced party which landed at Kimpo airfield near Seoul on 4 September 1945 and another small advanced party, in this instance 14 men of Major General Archibald V. Arnold’s 7th Division arrived be sea at Inchon on 8 September, and the main landing began on the following day.
Primary responsibility for US interests in the Korean peninsula was vested in General Douglas MacArthur, who was already overburdened with his involvement in the occupation of the Japanese home islands and therefore devolved the task of imposing and controlling the occupation of Korea to Hodge. The commander of the XXIV Corps established his headquarters at the Banda Hotel in Seoul, ordained a military administration, declared English to be the official language of Korea, and began the process of building an independent Korean government friendly to the USA. A good battlefield commander, Hodge was a poor diplomat who disliked the Koreans and was ignorant of Korean culture and how this differed from Japanese culture. Hodge therefore committed many egregious errors, including an instruction for his men to 'treat the Koreans as enemies'. Moreover, a shortage of manpower meant that Hodge allowed the old Japanese police force to remain on duty for crowd control and similar work, and also kept the Japanese colonial administration, if only in the short term, until he could find suitable US replacements.
Following a Korean complaint, however, the US military government in Tokyo officially removed Korea from Japan’s administrative control on 2 October, though many Japanese officials in Korea were retained as advisers to their US replacements.
The overall effect of these early stages of 'Blacklist Forty' had the effect of persuading Koreans that the Americans were not liberators but rather occupiers.
Preparations for the withdrawal of US and Soviet forces from Korea could not start before the USA and USSR had agreed on the establishment of a unified Korean government, as had previously been agreed, that was supportive of both nations' interests. However, the Soviets refused to accept any idea that did not involve the creation of a communist state, and the negotiations were therefore fruitless.
The USA therefore sent the 'Korean question' to the United Nations Organisation. It was in September 1947 that the UNO agreed to accept the challenge, and proceeded with of UN-supervised elections. The USSR made it clear, however, that any UN decision would be applicable only to the portion of Korea south of the 38th parallel, and that anything north of the parallel would be determined by either itself or the new Democratic People’s Republic of Korea, which became generally known as North Korea.
The elections were held, and Syngman Rhee became president of the new Republic of Korea, generally known as South Korea, on 24 July 1948. The US and Soviet occupations of Korea ended soon after this, leaving the Korean peninsula divided and in the early stages of a political ferment which, wholly unforeseen by the Americans, led to the outbreak of the Korean War (1950/53).