What was the long-term significance of the Korean War?
For Korea, of course, the long term significance of this war was that the peninsula remains divided and that there continues to be deep animosity between the two countries.
Outside of this, the major significance of the war was that it helped to create the basic structure of the Cold War. It helped to cement the idea that the US would fight to protect countries that were in danger of being overrun by communism. This would commit the US to help many countries that we might not have helped otherwise. The most important example of this was Vietnam.
So, the Korean War helped to commit the United States to intervening all over the world to try to contain communism.