Consider a promise not to use nuclear weapons. To the extent that this promise is credible, it’s destabilizing, because it rewards a first strike. It gives an opponent an incentive to act first, and to act before someone else takes power and changes the policy.
Well, how about “no first use,” a stated policy to only use nuclear weapons in retaliation? This has no moral or rational basis. Why, having suffered a nuclear strike, would we inflict one, when it could do no good?
The sensible and moral policy is to say, and show, that we are prepared to use nuclear weapons against legitimate targets if a sufficiently grave threat to our national interest requires it. Furthermore, we should hope this is the policy of all the nuclear powers. That’s how you get stability. If every power is reluctant but willing, and ever power has delivery systems that can survive a first strike, every power has a powerful incentive not to strike first.
Unless they’re religious fanatics hoping to bring about the end times. People like that can’t really be deterred.