Acheson and Truman
Truman believed that the US needed to respond to the invasion. The US was able to get a UN resolution passed calling for the North Korean withdrawal. He ordered assistance be given to the South Koreans.
Washington first learned of the attack in Korea at 9:04 Saturday night, when the UP called the state department to confirm that an attack had in fact taken place. President Truman was home in Independence, Missouri for vacation when the war began. Secretary of State Dean Acheson first notified him by phone at 9:20 Missouri time. He told Truman: "I have very serious news. The North Koreans have invaded South Korea." Truman believed from the moment he heard the news that this might be the opening round of WW III. Truman approved of getting a vote from the Security Council condemning the attack. As the word from Korea worsened, Truman hastened back to Washington. On the way, Truman reviewed his options and concluded that he would not allow another "Munich" to occur on his watch. If Hitler had been stopped in Czechoslovakia, maybe WW II would not have occurred; thus, if WW III was to be averted, the Communists must be stopped in Korea. His thinking, which was mirrored by his advisors, was that the Soviet Union was behind the attack. The United States was successful, thanks to the Soviet boycott of the Security Council, in obtaining a resolution calling for the North Korean withdrawal. Truman gave the green light first for speeding arms to South Korea and then using the Airforce to attack the North Koreans in the South. Under Acheson's direction, the US went back to the UN and had the Security Council vote on a resolution which called on member states to "furnish such assistance as may be necessary to repel the armed attack and to restore international peace and security in the area." Thus, the US was armed with authority to act on behalf of the UN to intervene in the Korean conflict.