Syag, yes, good points. I think in psychology they have words for this kind of thing, like confirmation bias or egocentric bias. There’s also an innate tendency to see people from other groups as all the same, while appreciating all the diversity within one’s own group.
And whenever there’s a sense that one’s group is under attack from the outside, people tend to revert to hard-core group loyalty, “My country right or wrong” and such, whether or not they’re right.
Germany even claimed that it started WWII in self-defense, as crazy as that sounds (perhaps because people can’t help but rally around their leader in times of external threat).