At the end of World War II, the US stayed in South Korea for a while to keep order while the country started to recover from being colonized by Japan. The US also was there because it wanted to keep the Soviets from taking any more of Korea than they already had. While there, the US put a man named Syngman Rhee in power.
But then the US left in 1949. Soon after, in 1950, Dean Acheson gave his speech saying that South Korea was outside the US defense perimeter. This helped cause the Korean War.
When that started, the US became the main protector of South Korea. The US fought off the North Koreans (and later Chinese). After the war, the US left troops in South Korea to protect the country from further attacks.
After World War II, Korea was split into two nations, North Korea ruled by Kim Il Sung and supported by USSR while South Korea was ruled by Syngman Rhee and supported by the US. When Communist took over China, the U.S. started to prepare for possible war in order to defend South Korea since it was a strategic point near USSR. With US aid (sanctioned by UN), South Korea was able to push back the North Koreans to the original divide line, however the US wanted to completely unite Korea, threatening China’s national security. Thus, China sent troops to push the US back to the 38th parallel again. This military stalemate proved detrimental to Truman, especially after he dismissed General MacArthur, war hero of World War II and the Korean War. Truman’s failed promises for a swift, democratic victory in Korea led to skepticism towards Democratic solutions to the Korean War, the nuclear arms race, and the Cold War in general. Thus, the Republicans were able to win the election of 1952 with Dwight D. Eisenhower, who promised an end to the war through peaceful means.