> he is considering a policy known as “sole purpose”, which would make clear the US would use nuclear weapons only in a narrowly-prescribed set of circumstances

> he is considering a policy known as “sole purpose”, which would make clear the US would use nuclear weapons only in a narrowly-prescribed set of circumstances — such as to deter a direct attack on the US, or to retaliate after a strike. Couldn't the "set of circumstances" include retaliating after an attack on an ally by a nuclear-armed nation (or alliance)? Naively we might want a global "no escalation" treaty/convention/commitment, which would compel countries to not use nuclear weapons to respond to conventional attacks, but that would make it very hard for the US to protect Taiwan against invasion by China, or SK from invasion by NK, or Estonia from Russia, or Israel from Iran. There are perhaps other strategic goals of the US nuclear umbrella, but a commitment to only using nukes against nuclear-armed adversaries would at least give countries some incentive to remain non-nuclear.