Trying to prevent the proliferation of nuclear weapons is probably futile. This is technology from World War 2. Imagine if the nuclear-proliferation regime had instead tried to prevent the spread of computing or rocketry. Surely development of those technologies would have occurred more slowly, but each advance making computers or rockets more affordable and reliable would have made it easier for marginal organizations to start their own projects. Hell, in 1911, no one in the world knew about nuclear fission. Now an interested schoolboy can tell you that fissile material + critical mass + pressure vessel = boom. Enrichment seems to be the bottleneck in the production of nuclear weaponry; if it’s getting easier, that’s bad news for anti-proliferation efforts.
The threat of mutually assured destruction worked pretty well during the Cold War, but both sides possessed not only staggering nuclear arsenals but massive conventional forces as well. A future nuclear conflict could involve a smaller country using nuclear weapons to defend itself from an invader. It might not work.
The truth is that nuclear weapons are not so devastating as might be imagined. Tactical employment of nuclear weapons won’t automatically result in victory; armored vehicles provide excellent protection against the blast (though not the radiation) effects of an atomic bomb, even if you know exactly where the target is. Terrain also lessens the effects of a nuclear bomb; this is one of the reasons (not the only) the United States did not use nukes along the mountainous 48th parallel. Not every possible nuclear target is a paper city on a plain. Invading armies would still sustain higher casualties, but I doubt the difference would be any worse than the proliferation of gunpowder. The armies of Napoleon or Grant could keep fighting; future armies could do so again. The burning of cities was once a commonplace feature of war, and could become so again.
The day is coming when someone uses an atomic bomb. The world will not end. Those on the business end will be shocked and amazed. Then they’ll keep fighting.