Americans became involved in the Korean conflict when the United Nations (UN), only five years old, called upon member countries to give military support to South Korea, which had been invaded by troops from Communist-ruled North Korea on June 25, 1950. The United Nations considered the invasion to be a violation of international peace and called on the Communists to withdraw. When they did not, 16 countries sent troops and some 40 countries sent supplies and military equipment to the aid of the South Korean armies. About 90 percent of the UN aid came from the United States. But North Korea received aid too—the Chinese sent troops and the Soviet Union provided equipment for them to sustain the war, which lasted until July 27, 1953. After three years of fighting, an armistice was called, but a formal peace treaty was never drawn up between the neighboring countries, prompting the United States to maintain military forces in South Korea in an effort to discourage any further acts of aggression from the north.