Here's how the Korean War started:
When 90,000 North Korean troops crossed the 38th Parallel and attacked South Korea on June 25, 1950, the United Nations also went into action. An emergency session of the UN Security Council was held -- from which the Soviet Union was absent because the USSR was boycotting the UN over the exclusion of communist China from the organization. Truman announced to the American people that he was authorizing sending US troops to prevent South Korea from being overtaken by communism. The UN Security Council met again and approved a US resolution approving the use of force against North Korea. Military forces in the Pacific theater, based in Japan, were deployed in the effort. There was no formal declaration of war by the US Congress, but Congress did vote to extend the draft and also authorized the president to call up military reserve personnel for duty.
The Korean War was an effort led by the United States to keep South Korea free and democratic. The Korean War lasted from 1950 to 1953, and about 5 million people (soldiers and civilians) lost their lives in the conflict. Korea remains divided at the same line where things stood prior to that war.