The Korean War began when the North Koreans invaded South Korea. For the most part, the North Korean People's Army ( NKPA) rapidly pushed through South Korea, forcing the South Korean forces to a small area known as the Pusan area. The United Nations declared North Korea's attack an act of aggression and, just 2 days later, appointed 15 other countries under the command of the United States to fight the northern enemy. US president Harry Truman placed General Douglas MacArthur supreme commander of the US and UNs forces.