Why did the US go into the Korean war?

Answer 1

The attacks by North Korea on South Korea were seen as a proxy effort by China and the USSR to extend the influence of communist states. The US was pursuing a policy of containment to limit this spread.

When the UN condemned the invasion, the US and other UN members sent forces to assist the South. Eventually this led to a prolonged war (1950-1953) between forces of the US and China in Korea.

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Under the "domino theory" any spread of communist influence, as in Korea, could lead to the loss of Japan or other Asian countries. The US saw the North Korean offensive as a move to expand Soviet influence, and acted in order to forestall aggression elsewhere in the world. The criticism of Truman as being 'too soft' on Communism after the fall of China gave political impetus to US involvement as well. A similar scenario led to the massive US intervention in Vietnam in the 1960s.

Answer 2

Beacuse North Korea was Communist and US did not want Communism invading more land.