I'd guess there's an evolved component which exhibits among sufficiently intelligent humans living in relative peace and prosperity for a long time. An environmentally-modulated trait. Otherwise there wouldn't have been a toehold for subversives to cling to and start prying at the seams in the first place. With a homogenous population, that would be fine. They wouldn't need a particularly strong in-group bias - only enough to maintain culture and borders. Mix in other tribes with their own self-preservation tendencies and the host nation's kindness becomes a weakness. Tribalism is back on the menu as a form of defense.
I think you’re underestimating the power of 60+ years of total indoctrination coming from education, news media, and entertainment. I don’t think an ethnic/racial cohort requires a particular vulnerability when subjected to that level of attack.
I'd guess there's an evolved component which exhibits among sufficiently intelligent humans living in relative peace and prosperity for a long time. An environmentally-modulated trait. Otherwise there wouldn't have been a toehold for subversives to cling to and start prying at the seams in the first place. With a homogenous population, that would be fine. They wouldn't need a particularly strong in-group bias - only enough to maintain culture and borders. Mix in other tribes with their own self-preservation tendencies and the host nation's kindness becomes a weakness. Tribalism is back on the menu as a form of defense.
I think you’re underestimating the power of 60+ years of total indoctrination coming from education, news media, and entertainment. I don’t think an ethnic/racial cohort requires a particular vulnerability when subjected to that level of attack.