After several years of increasingly bloody frontier incidents along the 38th parallel, the Republic of Korea was invaded by the North Korean Peoples' Army on 25 June 1950. Despite earlier indications, the Pentagon was caught off-guard. As the North Koreans swept south, overwhelming all opposition, the US called on the Security Council to invoke the United Nations Charter and brand the North Koreans as aggressors. This was done and member states were called on to send in military assistance. The first American troops were then sent in to stiffen resistance against the invader. The British government responded at once and elements of the Far East Fleet were soon in action along the Korean coast, together with ships of Commonwealth navies.

No one knows exactly how many people died in this war. In a sense it was a civil war fought out with foreign participation on both sides. It was the first military test of the United Nations and also the last martial adventure of the old Commonwealth. The American Department of Defence acknowledges that almost 40,000 of its servicemen died, either in battle or of other causes. British casualties were 1,078 killed in action, 2,674 wounded and 1,060 missing or taken prisoner.