This question already has an answer here:
Proponents in country A which has nuclear weapons support their existence by using the 'deterrence' argument. However, proponents in country A actively attempt to prevent country B from developing their own nuclear weapons.
Think USA and the western world versus Iran, North Korea, and earlier Pakistan, etc.
This seems contradictory, does it not? If the existence of nuclear weapons in country A prevents other countries from engaging in warfare against country A, then the same logic applies to country B: existence of nuclear weapons in country B would prevent other countries from engaging in warfare against country B.
Assuming that world peace and avoiding unjust warfare is what we all want, surely this is a good thing for both country A and B.
Thus we can conclude from country A being opposed to country B's nuclear development plans that world peace and avoidance of unjust warfare is not a priority for country A, and may in fact be counterproductive to their real goals.
Am I correct in my reasoning?
A minor side-argument made by country A may be that if country B develops nuclear weapons, they may give them away to some third-party, who could use the nuclear weapon against country A, such that country B is not a main suspect. This fairy tale seems worrying until you come up with the obvious solution: international oversight. Allow country B to have and control as many nuclear weapons they want, but they must always remain under international oversight, so that any actions taken by country B in regard to those nuclear weapons can be independently investigated. The same would of course apply to nation A and all other holder of nuclear weaponry. Problem solved.