The North Korean invasion initially was very successful. By early August 1950, UN forces held only the southeast corner of the Korean peninsula. The tide began to turn in September as General Douglas MacArthur led an American amphibious invasion at In'chon. From this point, UN troops drove the North Koreans back across the 38th parallel.
General MacArthur strongly and openly advocated the pursuit of complete victory over North Korea. Under his leadership, American forces penetrated deep into enemy territory. By the end of October, the war appeared to be nearly over. The Canadian government even temporarily reduced its commitment of troops in anticipation of this outcome. Such optimism, however, proved unjustified.
MacArthur's insistence on following the North Koreans north of the 38th parallel was extremely controversial. In fact, by ordering the pursuit of North Korean forces, MacArthur had contravened the orders of US President, Harry S. Truman. Even more importantly, MacArthur's actions led the Chinese to intervene on behalf of North Korea. The involvement of Communist China completely altered the nature of the war. Late in November 1950, the Chinese launched an assault that drove the UN armies back. Another Chinese offensive in January 1951 forced a further UN withdrawal.