With advisement from the Soviets and after several small conflicts with the South, North Korea invaded South Korea in June of 1950, and the UN immediately responded. The United States requested to the Security Council that they send troops to aid the South Koreans, and they accepted. (The decision was not vetoed by the Soviets, luckily, because of a Soviet boycott of the UN for their refusal to recognize China as a communist country.) World War II hero General Douglas MacArthur was put in charge of these troops, and 90% of them were American. The “UN” troops pushed the North Korean forces all the way from the southern tip of South Korea to the North Korean border with China.
Feeling threatened, China got involved and pushed the UN forces back to the 38th Parallel, the dividing line between North and South Korea. The war ended in an armistice there in 1953, and the United States considered it a victory.
Although the North became communist after a “free” election, they had successfully contained another Soviet expansion effort. It is commonly known as the forgotten war, due to the fact that it went virtually unheard of in the United States, gaining little media coverage.