Zombie Cliché Lookout:

In many zombie stories, the authorities reach a crisis point where they need to make a big stand and push back the ever-growing army of the living dead. Because the writers of zombie stories typically have a pretty negative view of the competency of those in authority, this big stand is almost always doomed to fail. The result is a severely weaken, if not completely destroyed defense network. This is a very important trope in the zombie genre, as it forces the protagonists to rely only on themselves and possibly others in their party. Of course mistrust, power struggles, competing love interests, and one-upmanship generally erode the group’s strengths shortly before the final act.