The history of South Korean as an political identity began when the Northern and Southern parts were divided in 1945. At the close of World War II and the beginning of the Cold War, Korea was under dual control, with the United States controlling the Southern part and the Soviet Union controlling the North.
As quickly as the second world war ended the Cold War began trapping Korea in its midst with both the Soviet Union and the United States supporting ideologically friendly governments around the world. This situation led to the eventual split into the pro-communist North Korea and the America-leaning South Korea. Hence, from its inception South Korea and the United States have enjoyed a close relationship.
The situation escalated when North Korea invaded South Korea in June 1950 to unify the island starting the Korean War which lasted until 1953. The North initially overwhelmed the U.S. funded and trained South, which had no tanks, offensive airpower or anti-tank weaponry. The North also quickly occupied various key southern cities and looked like it would have little resistance in capturing the entire peninsula. The United Nations then intervened.
United Nations Security Council Resolution 82 authorized the intervention of U.N. forces which were mainly composed of American troops alongside 15 other countries. With this help the South managed to recover all of its occupied territories. Encouraged by this success, U.N. forces continued into the North to try and unify the peninsula under the Southern command, which led to Chinese intervention in the war. Fighting lasted until the stalemate and ceasefire of July 1953 and the establishment of the demilitarized zone.
In order to guarantee the South's security from future North Korean aggression, the United States and South Korea signed the Mutual Defense Treaty in 1953. The treaty stipulated that either country would come to the defense of the other if attacked by a third party. Additionally the American and South Korean military to this day enjoy a high level of cooperation with the United States stationing approximately 37,000 troops in South Korea.
Other Avenues of Cooperation:
Coupled with the military cooperation the United States helped South Korea transform its economy into one of the most vibrant in Asia. Both countries remain close trading partners symbolized by official figures which rank Korea among the top 10 U.S. trading partners. Both countries have also cooperated in the six-party talks, a series of negotiations with North Korea in hopes of resolving the North Korean nuclear issue.