The Korean War was a military conflict between North Korea, supported by the Soviet Union and the Communist China, and South Korea, supported by the United Nations, primarily the Unites States. On June 25, 1950, North Korea invaded South Korea. An armistice was signed on July 27, 1953, without a decisive victory. Though called the "forgotten war," both sides are still, technically, fighting.
The United Nations' decision to intervene militarily in Korea is a major turning point in history, as the organization shifted from a peacekeeping to a military role. This was a clear break from the League of Nations that proceeded it, and it allowed for the possibility of future military interventions in the name of peace.