South Korea–United States relations
Following Japanese surrender at the end of the Second World War, the Allied Powers divided the Korean peninsula along the 38th Parallel with two occupation zones with the United States in the south, and the Soviet Union in the north that would later become North Korea. On August 15, 1948, the southern half of Korea declared independence as the Republic of Korea with Syngman Rhee as president. The United States recognized the Republic of Korea as an independent state on January 1, 1949 and diplomatic relations between two countries were established on March 25 in the following year.
Cross-border skirmishes and raids at the 38th Parallel escalated into open warfare when the North Korean forces invaded South Korea on June 25, 1950. The Korean War broke out when North Korea invaded South Korea. In response, 16 member countries of the United Nations, including the United States, came to the defense of South Korea. It was the first significant armed conflict of the Cold War with extensive deployment of American and other troops.
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.