The "us" vs "them" thought pattern is the essence of "Western" (read: white, European, colonialist) culture, dating back to Plato's Republic.

Marimba Ani's seminal work Yurugu is a sociological study of this culture from a non-western perspective. I can't recommend it strongly enough if you want to understand the nexus of social/psychological/epistemic factors that sustain the "us" vs "them" thought pattern and its corresponding cultural manifestations.

https://archive.org/details/yuruguafricancen0000anim/page/n3...