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.
38th parallel north
Japan had occupied the Korean peninsula between 1910 and 1945. When Japan surrendered in August 1945, the 38th parallel was established as the boundary between Soviet and American occupation zones. This parallel divided the Korean peninsula roughly in the middle. In 1948, this parallel became the boundary between the Democratic People's Republic of Korea (North Korea) and the Republic of Korea (South Korea), both of which claim to be the government of the whole of Korea. On 25 June 1950, after a series of cross-border raids and gunfire from both the Northern and the Southern sides, the North Korean Army crossed the parallel and invaded South Korea. This sparked a United Nations resolution against the aggression and the Korean War, with United Nations troops (mostly Americans) helping to defend South Korea.