On February 22nd, 1946 George Kennan, the Deputy Head of the American Mission in Moscow sent a coded message to Washington D.C. which became known as the Long Telegram. In this analysis of Soviet expansionist ambitions Kennan recommended that the U.S. and it's allies must resists Soviet pressure wherever it was exerted in the world. This policy was called the Doctrine of containment and predicted that if Soviet growth were contained within it's current boarders it would eventually recede and the Soviet threat would shrink.
On January 12, 1950 the American Secretary of State, dean Acheson, in a speech to the National Press Club, seemed to fail to include South Korea as a part of the vital defence perimeter for the United States. On the 25th of June, 1950 a massive surprise attack was launched by against South Korea. Although it is still debateable, many believe that the Soviet Union and China believed that the U.S. would not respond militarily.
The Soviet Union had decided to boycott United Nations Security Council meetings in January of 1950 as a protest against a Security Council seat being held by the Republic of China or Taiwan, rather then the people's republic of China, the Soviet Communist alley. Due to their absence during the debate on the Korean invasion, the U.S. and the other western powers were able to pass UNSC resolution 82 which accused North Korea as being the aggressor in the conflict and offered U.N. military support to South Korea. president Truman ordered immediate military help to South Korea on June 27th.
On June 28th Lester Pearson, Canada's ambassador to the United Nations voiced his support of the resolution. Canada stepped up to offer 3 destroyers and an air transport squadron for the United Nations force being assembled to defend South Korea. This was later increased to a larger land force which began with the 2nd battalion of the PPCLI. By the end of the war in 1953, South Korea had been defended and the United Nations had regained most of the territory initially taken by North Korea. Of the 21,940 army personnel and 3600 naval personnel who served in Korea 312 were killed in action, and 1202 were wounded.
Although the Soviet Union would insure that they were no longer absent form Security Council Meetings, the principle of collective security with the United States in the lead was established. The Canadian ambassador, Lester Pearson would use this Experience to design a plan to halt military action in the middle east through the intervention of UN troops in 1957, for which he would later be awarded a Nobel Peace Prize.