Wow.

Well first off the Korean war started when North Korea invaded the South and the US came to the aid of the south. Second, the only time that the US military (General MacArthur) contemplated using nukes against China was during the Korean War and this basically led to Truman firing MacArthur - and this was before China had nukes.

And this is the most important point. NK does not need nukes as the US is never going to invade it. Why would they - SK is rich and NK is poor - what then does NK have that the US would want? That being said, ironically, NK having nukes does risk a massive response, if NK gets a little 'careless'.