One thing to remember is that a major goal of fusion research is also weapons applications, those that is never publically admitted.
Or maybe you had completely different weapon applications in mind?
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).
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.
Could you give some more detail? What form would a fusion weapon take? Is it about the laser technology?
The NIF experiment explained here, which uses a laser to generate X rays by heating the walls of a heavy metal hohlraum, and then uses these X-rays inside the hohlraum to compress a pellet of gas to ignite a fusion reaction is very similar to the conditions inside an H-bomb, which also generates X-rays (via a fission reaction) to compress and ignite a fusion reaction.
The weapons use is relatively simple - fusion reactions expel much more energy than fission reactions, and using the power of a fission reaction to compress and to ignite a fusion reaction inside a large amount of gas expels much more energy than simply exploding the power of the fission reaction outwards.
JET, ITER, Wendelstein and other magnetic confinement facilities on the other hand are investigating fusion as a source of grid energy.
Having relatively easily handled, high density energy sources would also probably lead to deployment of energy weapons.
