[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.