History of South Korea
On August 15, 1948, the Republic of Korea was formally established, with Rhee Syngman as the first president. With the establishment of Rhee's government, de jure sovereignty also passed into the new government. On September 9, 1948, a communist regime, the Democratic People's Republic of Korea (North Korea), was proclaimed under Kim Il-sung. However, on December 12, 1948, by its resolution 195 in the Third General Assembly, the United Nations recognized the Republic of Korea as the sole legal government of Korea.
United States involvement in regime change
As the Empire of Japan surrendered in August 1945, under the leadership of Lyuh Woon-Hyung committees throughout Korea formed to coordinate transition to Korean independence. On August 28, 1945 these committees formed the temporary national government of Korea, naming it the People's Republic of Korea (PRK) a couple of weeks later. On September 8, 1945, the United States government landed forces in Korea and thereafter established the United States Army Military Government in Korea (USAMGK) to govern Korea south of the 38th parallel north. The USAMGK staffed the governing administration with Japanese governors and many other Japanese officials who had been part of the brutal Japanese imperial colonial government and with Koreans who had collaborated with it, which made the government unpopular and engendered popular resistance. USAMGK refused to recognize the PRK government, which had been formed to self-govern the country, and the Provisional Government of the Republic of Korea, which had been based in China during WWII and had fought against the Japanese, and then the USAMGK by military decree outlawed the PRK government. In October 1948, USAMGK sent units to attack Koreans who were seeking Korean independence, and carried out several mass atrocities, including the killing hundreds of Korean civilians on Jeju Island who were suspected of supporting those in favor of independence.