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.
Korean Peninsula
Until the end of World War II, Korea was a single political entity whose territory roughly coincided with the Korean Peninsula. In August 1945, the Soviet Union declared war on Imperial Japan, as a result of an agreement with the United States, and liberated Korea north of the 38th parallel. U.S. forces subsequently moved into the south. By 1948, as a product of the Cold War between the Soviet Union and the United States, Korea was divided into two regions, with separate governments. Both claimed to be the legitimate government of all of Korea, and neither accepted the border as permanent. The conflict escalated into open warfare when North Korean forces—supported by the Soviet Union and China—moved into the south on 25 June 1950. Since the ended the Korean War in 1953, the northern section of the peninsula has been governed by the Democratic People's Republic of Korea, while the southern portion has been governed by the Republic of Korea.