What he actually asked was that, if the US detected the preparation for the launch of a tactical nuclear strike, that they should launch a preemptive strike to prevent the attack rather than wait and do so in retaliation. Thus saving the lives of everyone in the target area.
A reasonable request, but not one which, unfortunately, would seem politically acceptable.
Having said that, preemptive self-defence is a legally sanctioned US concept, just not sure how it would extend to cover Ukraine - unless you argue that the USA would inevitably be drawn in after the nuclear threshold was passed.
https://digitalcommons.law.villanova...15&context=vlr