In the early morning of 25 June 1950 local time, North Korean forces attacked across the 38th Parallel. Equipped with Soviet-made tanks, supported by massed artillery fire, the communist offensive quickly drove south through Koesong and toward Uijongbu north of Seoul. Other attacks pressed against the mainly South Korean defenders all across the frontier to Kangnung on the east coast.
As the news worsened over the course of the day, General MacArthur, Commander in Chief, Far East Command, then at General Headquarters in Tokyo, ordered the shipment of ammunition from depots in Japan to resupply the South Koreans. That was as far as he could proceed on his own authority.
President Truman, then at home in Independence, Missouri, heard the news of the invasion on the night of 24 June from his secretary of State, Dean Acheson. The President then tasked Acheson to request a meeting of the United Nations Security Council. The UNSC met at 1400 the following day and, owing to the continued absence of the Soviet representative (who had boycotted that body since January), the Security Council passed a resolution calling for peace and the restoration of the 38th Parallel boundary, calling upon “all members to render every assistance to the United Nations in the execution of this resolution and to refrain from giving assistance to the North Korean authorities.”
On the same day, as President Truman traveled back to Washington, the Joint Chiefs of Staff alerted MacArthur to the possibility, should the United Nations ask member nations to employ military force, that he might be directed to commit air, naval and ground forces to stabilize the combat situation.
The key question facing the United States, and indeed the United Nations as a whole, was the reaction of the Soviet Union. The American intelligence community believed the Soviets were the true aggressors in Korea, in spirit if not in fact. A central Intelligence Agency (CIA) report on 19 June had called the Democratic People’s Republic of northern Korea “a firmly controlled Soviet Satellite that exercises no independent initiative and depends entirely on the support of the USSR for existence.”
The U.S. embassy in Moscow argued in their daily summary report that the attack was “a clear cut Soviet challenge to the United States which should be answered firmly and swiftly because it constitutes a direct threat to US leadership of the free world against Soviet-Communist imperialism.”
In Washington on the evening of 25 June, President Truman met with his senior civilian and military advisors at Blair House. During the resulting discussions, which focused on the communist threat to Korea as well as Formosa, the prevailing opinion was that the United States needed to draw the line against Soviet aggression somewhere, and that somewhere was Korea.
Based on multiple intelligence reports over the previous half year, President Truman believed the Soviet Union was “not willing to undertake a global war at this time,” but was merely testing American determination.
Based on this meeting, President Truman ordered the Seventh Fleet to the waters off Korea and Formosa as well as Air Force jets and aircraft to the region, mainly to prevent the Inch’on area from being overrun to ensure the safe evacuation of American nationals. This decision was made without sanction of or reference to the United Nations.
On 27 June, with the North Koreans about to seize Seoul, President Truman authorized air and naval forces to engage in combat operations in support of the Republic of Korea.