[TeMPOraL]

Have you heard of brinkmanship?

A hypothetical country threatening NZ with nukes in this example would be presenting an argument: either let us in, or else we will nuke you - in hopes that NZ prefers being overcrowded but surviving, over being glassed. NZ will not take the deal - at first. The other country can insist more and more, until the situation is so tense that any random event could trigger launching the missiles.

And then, if NZ acquiesces and lets people from one country evacuate to their land, another nuclear country would ask for the same deal. Or they may even point half the nukes at the first country early on, to make sure that either they get the deal or no one else.

Thing with brinkmanship is that at no point anyone wants to actually use the nukes - but at every point it's locally optimal to up the threat, and there is no obvious stopping condition until someone trips over a cable, hits a launch console, and the nukes starts flying.