[hackinthebochs]

No, that's not how dynamical feedback systems work. Border expansion is held in check by the opposing force from the territories being expanded into. When one country can't protect their borders, the over-extended regions are likely to eventually get annexed up until the point where the country's force can provide sufficient opposition. That's the way of the world.

Nuclear weapons don't change this calculus significantly, except for the fact that a country with nukes can now defend their borders even with an incompetent conventional force, making expansion into a nuclear power negative-sum. Nuclear weapons do not make it easier to annex new territory as they are practically useless tactically. So there is no scenario where a fascist with nukes takes over the world.

What nukes do is massively raise the costs of intervention by stronger conventional forces. For the same reason it is negative sum to annex territory from a nuclear power, it is negative sum to interfere with the core interests of a nuclear power. If Putin judges Ukraine as key to Russia's survival, he may judge the cost of retreat to be similar to how he would judge encroachment on Russian territory, thus making the use of nuclear weapons rational. What nuclear weapons do is force the recognition of the core interests of your adversaries. There is a stable state where the core interests of Russia are recognized, with any further expansion having negative cost. The world will get to that point one way or another. Endless ratcheting of tension in dynamical systems cannot be contained forever.