[tsimionescu]

No country in the world that doesn't already have them can "just build nuclear weapons if it wanted", international treaties are extremely prohibitive of that. In particular, if Japan wanted to build a nuclear weapon, it would face tremendous opposition from China, while S Korea would face the same from both China and N Korea.

Also, fission weapons tests are currently banned, which also indirectly bans fusion weapons tests, as fusion weapons require a fission step. ICF experiments perfectly mirror conditions inside a fission-fusion bomb though, so are a way of furthering research into improving the yields or other characteristics of such bombs (which have existed since the early 1950s, this is not some far-fetched concept).

[myrmidon]

> international treaties [...], political opposition

Are just reasons for countries to not want nuclear weapons. But if e.g. Japan announced a nuclear weapon program (or it was uncovered by foreign intelligence) I highly doubt that anyone could/would stop them-- hinder their economy or facilities with sanctions or sabotage, sure, but outright stop them from becoming a nuclear power? Unlikely.

IMO every bigger western-ish nation state has the ressources required to build nuclear weapons; they just lack motivation to do so.

Just consider that smaller states like France were already able to do this in the 1950ies, and relevant helper technologies (computer aided design, simulation, industrial control) are MUCH more accessible and advanced today.

Fusion research, on the other hand, is not going to help with the major challenge in becoming a nuclear power: it does not help with obtaining highly enriched fissile material.

If countries wanted to throw research funds at becoming a nuclear power, it seems infinitely more likely to me that those would go into "innovative reactor research" with the endgoal of producing HEU or plutonium.

[tsimionescu]

I agree with you that there are lower hanging fruit than fusion research for becoming a nuclear power.

I do not agree with you that Japan could in practice successfully acquire nuclear weapons with only economic sanctions to fear. I believe that China would be likely to declare war before allowing Japan to become a nuclear power, and I think it would have significant international support. Of course, that would be many years away, as a last resort if all else failed and their weapons program is nearing fruition. But I do believe it would happen eventually.