What was the Korean War about? And the purpose of it

The Korean War started when Kim Il-sung's (Communist) North Korean army crossed the 38th parallel to invade (Democratic) South Korea. The U. S. had a theory called the Domino Effect which basically theorized that if a certain amount of European countries became Communist then Europe would fall to the USSR's Communist Regime. So we sent our troops to aid in pushing back the North Koreans to the 38th parallel.