Credibility matters. One of the important aspects of having nuclear weapons is laying out the conditions in which you expect to use them, and a lot of internati

Credibility matters. One of the important aspects of having nuclear weapons is laying out the conditions in which you expect to use them, and a lot of international relations theory, especially in the military sphere, is about trying to analyze how you would expect states to respond to different situations. I don't think anyone seriously expects a nuclear state threatened with imminent destruction to forgo use of nuclear weapons (even in the presence of a non-first use statement). The question is how close to that condition does a state have to be in before it decides to use nuclear weapons. For the states with explicit non-first use policies, the expectation is that it is very close indeed. By contrast, Pakistan has effectively an explicit first use policy: it will resort to nuclear weapons very early in a war. The US has a particularly mealy-mouthed policy, saying that it will only use nuclear weapons against nuclear states, but heavy vacillation on the threshold of their use in such conflicts. All of the other nuclear states basically state that their nuclear weapons are solely a defensive measure.