From Foreign Policy, a couple years ago. A good reminder or introduction on the differences between realist, liberal, and constructivist perspectives on the international arena, couched in the context of how the zombie apocalypse plays out.
A structural realist would argue that, because of the uneven distribution of capabilities, some governments will be better placed to repulse the zombies than others.
A liberal institutionalist would argue that zombies represent a classic externality problem of… dying and then existing in an undead state and trying to cause others to do the same.
Constructivists would posit that the zombie problem is what we make of it. That is to say, there are a number of possible emergent norms in response to zombies.