Extended Deterrence
A nuclear-armed state's promise to use its arsenal, if necessary, in defense of an ally that does not itself possess nuclear weapons — the logic underlying the American nuclear umbrella over NATO allies. Faces a sharper credibility problem than direct national defense (the Cold War-era 'would you trade Boston for Berlin?' question), since a rational adversary might doubt a state would truly accept catastrophic risk to itself on another state's behalf. Ukraine, holding no formal collective-defense guarantee, faces this credibility problem in its most acute form.