On June 25, 1950, North Korean forces poured over the border separating the North and South parts of the peninsula, invading South Korea. This was considered a threat by the United States for two reasons: first, because the North Korean regime was Communist, and the Domino Theory was still widely believed; and second, because if South Korea fell, it would threaten American and allied forces in Japan.
Two days later, America announced that it would come to the aid of South Korea. Aside from the desire to oppose Communism, the Truman administration was keenly aware of the failures of appeasement at the start of World War Two, and did not wish to repeat this mistake.
In the end, the Korean War would last a little more than three years, cost nearly 4 million lives in total, and set the precedent for the Vietnam War – all for some very minor changes in the border between the two states.
By Signal Corps Photo – http://www.history.army.mil/photos/Korea/kor1950/SC343463.jpg, Public Domain, Link
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