In the beginning, the United Nations Temporary Commission on Korea was established in 1945 as a general world peacekeeping organization. This was set up to organize elections and built a democratic government. However, North Korea and the USSR refused to cooperate with the commission. On 25 June 1950 military forces from North Soviet Korea attacked the American South Korea. It seemed as a proxy war between the USSR and US, but in fact this attack was mainly decided by Kim Il Song, leader of North Korea, alone. With in a month, North Korean army controlled the whole peninsula apart from Pusan.
The Western Countries and US saw it as an invasion and expansion of Communism in Asia. UN called on North Korea to withdraw its forces from the South and voted to send assistance to defend South Korea on 27 June and the permanent seats in UN were the USSR, US, France, Britain and China.
USSR was busy fighting for the Chinese seat on replacement by Chinese Communist government. Therefore they were absent and couldn't veto on this decision. Totally 933,845 UN soldiers led by US general Douglas MacArthur was sent to Korea. 302,483 Americans, 2282 Australians, 6146 Canadians, 1389 New Zealanders, 14198 Brits, 5455 Turkish, 1700 Dutch, 44 from Luxembourg (Luxembourgens?), 1068 Colombians, 900 Belgiancs, 1271 Ethiopians, 1119 French, 1263 Greeks, 7000 Philippines, 826 South Africans and 1294 Thais.
The UN soldiers landed in September 1950. They pushed back North Korean armies within a month to the 38th parallel, general MacArthur decided to roll back them to China, therefore the fronts moved to River Yalu soon which was the border between China and North Korea. As soon as the UN soldiers arrived there, China joined the war and fought in the peninsula for about 3 years. Eventually, the...