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.
Fourth Republic of Korea
The Fourth Republic () was the government of South Korea between 1972 and 1981, regulated by the Yusin Constitution adopted in October 1972 and confirmed in a referendum on 21 November 1972. From 1972 to 1979, power was monopolized by Park Chung Hee and his Democratic Republican Party under the highly centralized authoritarian "Yusin System". With the assassination of Park on 26 October 1979, the Republic entered a period of tumult and transition under the short-lived nominal presidency of Choi Kyu-hah, controlled under severe escalating martial law and witnessing the coup d'état of December Twelfth, the violent unfolding of the Gwangju Democratization Movement and its armed suppression, the coup d'état of May Seventeenth and presidency of Chun Doo-hwan, and finally the transition to the Fifth Republic under Chun in 1981.