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.
South Korea
Historically, Korea had close relations with the dynasties in China, and some Korean kingdoms were members of the Imperial Chinese tributary system. The Korean kingdoms also ruled over some Chinese kingdoms including the Kitan people and the Manchurians before the Qing dynasty and received tributes from them. In modern times, before the formation of South Korea, Korean independence fighters worked with Chinese soldiers during the Japanese occupation. However, after World War II, the People's Republic of China embraced Maoism while South Korea sought close relations with the United States. The PRC assisted North Korea with manpower and supplies during the Korean War, and in its aftermath the diplomatic relationship between South Korea and the PRC almost completely ceased. Relations thawed gradually and South Korea and the PRC re-established formal diplomatic relations on August 24, 1992. The two countries sought to improve bilateral relations and lifted the forty-year-old trade embargo, and South Korean–Chinese relations have improved steadily since 1992. The Republic of Korea broke off official relations with the Republic of China (Taiwan) upon gaining official relations with the People's Republic of China, which doesn't recognise Taiwan's sovereignty.