<P> The Soviet Union challenged the legitimacy of the war for several reasons . The ROK Army intelligence upon which Resolution 83 was based came from U.S. Intelligence; North Korea was not invited as a sitting temporary member of the UN, which violated UN Charter Article 32; and the fighting was beyond the UN Charter's scope, because the initial north - south border fighting was classed as a civil war . Because the Soviet Union was boycotting the Security Council at the time, legal scholars posited that deciding upon an action of this type required the unanimous vote of all the five permanent members including the Soviet Union . </P> <P> Within days of the invasion, masses of ROK Army soldiers--of dubious loyalty to the Syngman Rhee regime--were retreating southwards or defecting en masse to the northern side, the KPA . </P> <P> As soon as word of the attack was received, U.S. Secretary of State Dean Acheson informed President Truman that the North Koreans had invaded South Korea . Truman and Acheson discussed a U.S. invasion response and agreed that the United States was obligated to act, paralleling the North Korean invasion with Adolf Hitler's aggressions in the 1930s, with the conclusion being that the mistake of appeasement must not be repeated . Several U.S. industries were mobilized to supply materials, labor, capital, production facilities, and other services necessary to support the military objectives of the Korean War . However, President Truman later acknowledged that he believed fighting the invasion was essential to the U.S. goal of the global containment of communism as outlined in the National Security Council Report 68 (NSC 68) (declassified in 1975): </P> <P> Communism was acting in Korea, just as Hitler, Mussolini and the Japanese had ten, fifteen, and twenty years earlier . I felt certain that if South Korea was allowed to fall, Communist leaders would be emboldened to override nations closer to our own shores . If the Communists were permitted to force their way into the Republic of Korea without opposition from the free world, no small nation would have the courage to resist threat and aggression by stronger Communist neighbors . </P>

Who was the head of the un troops defending south korea in the korean war