Us vs Them
Tribalism seems to be built into human nature. Maybe it's an adaptation of ancient man to aid in survival in a hostile world. Maybe it's a residual effect of a premortal war of good versus evil. Maybe it's just a result of the human brain doing what it does best (finding patterns in data sets).
Whatever the trigger or cause, we humans are quick to categorize, label, and group things. As a survival mechanism, it helps us make snap judgements. If threats are predetermined, we can react quickly to them.
One of the most used tactics of those who strive for power is to create unity through exclusion. We are the best, they hold us back. Distill any large scale human conflict down and at its root is "us versus them".
It's so ingrained into our human programming that it's almost unavoidable. This is what is so scary.
Before my oldest came out as a trans person, the LGBT+ community was not something I cared about. "Us" did not include them, thus they were "them", and whatever injustices or abuse that they endured was "too bad", but I didn't really care. I love my child and want what's best for him. And so now the LGBT+ community is "us" and the bigots that rail against us are "them", and man...what an eye opener that has been.
I can now see that things that used to be "totally reasonable arguments" or "just a joke", are really just disguised hate. There is certainly room for reasoned discussion, but where that lies on the spectrum is not where I used to think it was.
Race relations are another obvious example of this. I've actually lost friends over the protests this year. I thought any rational person would agree that systemic racism and police brutality are objectively wrong, but apparently in 2020 if you're in the cult you're "us" and if not, you're "them". And once you've been put in the "them" bucket and dehumanized, you're fair game to be maced, shot, and beaten.