The only genuinely hard part is the fissile material. It would be trivial for a country like Japan to do this in days/weeks (going from materials they have to

The only genuinely hard part is the fissile material. It would be trivial for a country like Japan to do this in days/weeks (going from materials they have to weapons); a country like, say, Sweden with less of a nuclear industry could also become a nuclear state fairly quickly. Plenty of companies or individuals could do it. The difference is -- if the US/Russia/etc. detect that someone is trying to make a nuclear weapon, all means up to an including a preemptive strike (nuclear or conventional) is on the table. Diplomatic pressure is the first step, but unless you're in a position to essentially blackmail/stalemate everyone, it would not run to completion. If, say, Nigeria were taken over by a crazed regime which wanted to make a nuclear weapon, and it didn't have either the US or Russia protecting it (or possibly China), all national means would be expended to prevent this from happening. (Stuxnet was a horrible Pandora's box opening, but IMO the US/Israelis behind it probably saved the world from a nuclear conflict, which is win/win for basically everyone. If it were a private individual or group, it would be even easier to go after them.) Theoretically, it may be possible to do "fission free fusion"; if someone can develop a reasonable, cheap, undetectable procedure for that, we are utterly fucked. The other thing which terrifies me would be laser separation/enrichment. Basically anything which could be done surreptitiously or in a widely dispersed way, vs. at a small number of detectable, centralized plants. (My money is on bioweapons destroying civilization, if not humanity, within 50 years, though, before nuclear weapons have a chance. Bioweapons are far easier to develop surreptitiously, and with very small scale initial capital requirements.)