Japan, South Korea, and nuclear weapons
Both Japan and South Korea possess sophisticated nuclear weapons capability, they’ve just chosen not to assemble any nuclear bombs.
This is especially true for Japan which has a huge stockpile of plutonium and could build a massive arsenal of nuclear bombs in less than a year. South Korea is a lot more limited in how quickly they could produce a large number of bombs.
Remember that nuclear weapons are 1950s technology (or 1940s technology if you are talking about simple enriched uranium bombs), and we are talking about countries producing the most sophisticated 21st century electronics like computer chips, LCDs, etc. The reason why countries like Iran find it so difficult to build a nuclear bomb is because their technology is still in the 19th century. Stuff that Iran has to sneak into the country, the government of South Korea can just order from Samsung or Hyundai.