Answer: Containment Policy
Explanation: He saw this as a threat to the containment of communism. This was because if the invasion succeeded and South Korea became a communist country, communism would be spreading instead of staying in one place; which defeated the whole purpose the containment policy was made. Truman's reaction was prompted by the belief that communism threatened the freedom and liberty of the American people. Communism spreading to East Asia also prevented a free trade economy and the upholdment capitalism.