What were the goals of the U.S. during the Korean War?
Although it is difficult to recreate the mindset in 1950, at that time the United States feared the Soviet Union would start World War III in a bid for global domination. The U.S. had just five years ago emerged from a very costly and destructive world war started by a regime with global domination on it mind, so naturally was still jumpy. Furthermore, in hindsight, people realized that if Hitler had been contained early, a world war might have been avoided. The U.S. was not going to make the mistake again of waiting too long to intervene.
So a chief goal of the U.S. was to avoid World War III by pushing back hard against what it understood as Soviet aggression. It wanted to discourage the USSR from expanding its empire—even if this meant going into a country quite remote from the U.S.
A more direct goal of the Korean war (the war was officially called a "police action") was to protect Japan, which the U.S. considered its linchpin to stability in Asia. Korea was too close to Japan for comfort, and the U.S. feared that a Korea unified as a satellite state of the Soviets could too easily try to invade Japan.
More specifically, the U.S. was determined to push the North Koreans back above the 38th parallel, the dividing line between North and South Korea, and keep them there. The goal was to nip communist expansion in the bud before it could grow into a monster that could only be contained by a major war.
The United States had some goals during the Korean War. The first goal was to keep South Korea from being taken over by North Korea. The United States wanted South Korea to remain a free and an independent country.
The Korean War occurred during the time when the Soviet Union was trying to spread communism. The United States had developed a policy to keep communism from spreading. This was known as the Containment Policy. The United States believed in the domino theory, which stated that if one country in a region became communist, then other countries in that region might also become communist. Thus, the United States wanted to prevent the spread of communism to South Korea. The United States was working to stop the Soviet Union from spreading communism to different places in Europe and in Asia. South Korea was one of those places.
The United States also believed it had to stand up to its main rival after World War II, which was the Soviet Union. The Soviet Union had broken some agreements it made with the Allies near the end of World War II. The United States felt that if the Soviet Union broke agreements and the United States did nothing about these actions, then the Soviet Union would continue to break agreements and would continue to try to spread communism to other places in the world.
The goals of the United States were to prevent North Korea from taking South Korea and unifying the two countries under a communist regime. This was part of the larger American policy of containment: to stop the spread of communism into areas where it did not exist.
The war began when troops from North Korea crossed over the 38th Parallel (which divided the two countries.) A secondary plan was to force American troops in South Korea to withdraw. President Harry Truman saw this as an attempt by the Soviet Union to spread its own influence as well as expand the communist bloc. In a speech to Congress, he stated:
The attack upon Korea makes it plain beyond all doubt that communism has passed beyond the use of subversion to conquer independent nations and will now use armed invasion and war.
The Korean War was actually a United Nations "police action," although the United States was a major participant. The plan was to force the North Korean troops back across the 38th Parallel and maintain the stability of the area. The war ended when President Dwight Eisenhower threatened to use nuclear weapons, and the Soviets (as well as the Chinese, who were now involved) backed down.