Imagine a near future where a hostile alien race successfully invaded the Earth with minimal effort because they figured out how antagonistic and competitive humans are. Just using patience and careful preparation, they applied some creativity to their "project".
Over a course of a few decades they spread some seeds of discord and just let it grow.
And humans did the rest.
The result: about a million surviving humans are being rounded and slaughtered by a large population of hostile aliens occupying and rebuilding our cities in ruins.
* * *
This is the environment in which a band of organized humans will accomplish the impossible.