United States, China, India, and Russia will never use nuclear weapons. These countries maintain a no-first-use stance simply because their conventional military capabilities are strong enough to protect their interests both within and beyond their borders.
Moreover, using nuclear weapons would defeat the very purpose of conflict—how can a country capture, control, or exploit resources if it has already destroyed them through nuclear strikes?
In fact, even if Iran possessed nuclear weapons, it is unlikely that it would use them in response to a conventional war, given the catastrophic consequences and limited strategic value of such a decision.