Not just nationalism, but tribalism.

That's why the US has such an appalling record of bias. Each new wave of immigrants (not to mention the ones that were here, to start with, and those that were dragged over, against their will) has been met with torrents of hatred, abuse, and state-sanctioned terror. In most cases, it had little to do with skin color, and everything to do with cultural differences.

When I lived in Africa, I saw tribalism writ large. It gets bad. If we think it's only in the developing world, then we simply need to look at what happened to Yugoslavia, after Tito died.

I think that tribalism is in our DNA, and it takes a real, concerted, conscious, effort to fight it. That's hard, humbling, work, and most folks aren't up for it.

Externally-imposed structure (Tito) works, but it's a temporary band-aid.