Why did the America enter the Korean War?

Answer 1

The Korean War was from 1950 to 1953 and that was during the Cold War. Cold War was basically about democracy and communism, and Russia and United States were the countries that had the most power on them. United States put their efforts on making Korea democratic while Russia tried to make Korea communist. So that was why United States joined as well as Russia even though neither of these countries fully made Korea democratic nor communist. (North, communist; South, democratic-- Korea divded into two.)

Answer 2

It was during the Cold War, when America's primary foreign policy was stopping the spread of communism. So when communist North Korea invaded non-communist South Korea, it was only natural that the U.S. would enter the war on South Korea's side.