In an all out war the countriDPRK nukes should be more than enough to neutralize both Japan and South Korea as US proxy allies against China. That means the US has to confront North Korea with nukes instead of China meanwhile the entire world stock market will tumble (hurt the US the most) each day. Even without nukes being used from either side with just conventional weapons would cause a lot of harm and distrust to both South Koreans and Japanese people whom sees that their alliance with the US has become problematic and dangerous.
In an all out conflict the North Koreans would take the opportunity to finish off the South and all US bases within the vicinity of China would be hit followed by Japan. China may not survive a conflict with the US but who is to know a weaken US may not get hit by the Russians doing an underbelly punch.DPRK nukes should be more than enough to neutralize both Japan and South Korea as US proxy allies against China. That means the US has to confront North Korea with nukes instead of China meanwhile the entire world stock market will tumble (hurt the US the most) each day. Even without nukes being used from either side with just conventional weapons would cause a lot of harm and distrust to both South Koreans and Japanese people whom sees that their alliance with the US has become problematic and dangerous.