War fought from 1950-1953, when the United States sent troops into North Korea to aid South Korea from its northern attackers. North Korea wanted to convert South Korea into a communist nation. The Norh Korean's were unsucessful due to the American forces being victorious in 1953. The whole war was fought to stop the spread of communism, and the United States did suceed in stoping the spread of communism into South Korea.
Basically, a war between two bitter rivals -- North Korea and South Korea. North Korea attacked South Korea, but with the help of the Americans South Korea was able to fend off the North Koreans. Then, the South Koreans attacked North Korea, but North Korea was able to fend off the South Koreans.