> The US and USSR/Russia did fight a good number of proxy wars throughout the cold war and beyond but those are distinctly different than the kind of all out war I think the original poster was referring to.
But they did fight them, including direct conflict with eachother. Which answers the upthread comment “If you have nuclear, why do you need anything else?” which was supported with the claim “Nukes prevent any war from happening between any powerful nation.”
Nukes don't prevent “any war” from happening between powerful nations, though they do seem to have some utility in restricting the escalation of such conflicts.
(Now, you could argue that unilateral conventional disarmament would leave no choice but to escalate to nuclear war, which would improve deterrence and prevent even small scale conventional, politically-covered attacks, but that argument would be highly speculative.)