supercat
Major
It would be a nightmare scenario for the U.S. if North and South Korea, and Japan all have nukes. There is no reason for the U.S. to station troops in East Asia anymore if that happens.If Japan gets nukes you can bet with almost 100% certainty the South Koreans will want them too.
At one point both those countries had covert nuclear weapons programs. They are both considered to be among the closest to breakthrough capability in nukes since 1990s.
I assume these "Japanese nukes" would be similar to US nuclear weapon sharing of B-61 bombs in Germany and Turkey. US is not interested in more countries getting their own nuclear weapons. They tried to kill the French nuclear weapons program for example.