In-group the Out-group
The social psychology principle of ingroup-outgroup bias indicates that we all make very arbitrary assignments of other people into either our ingroup (similar to us in some way) or outgroup (different from us in some way).
Once we have assigned someone to our ingroup, we are more likely to have feelings of trust, compassion, and unity toward them. Conversely, when someone is assigned to our outgroup, we are more distrustful, less compassionate, and have heightened feelings of aggression toward them.
Psychologists generally believe this is due to some primitive and deep-rooted tribalism which may have improved an individual’s chance at survival thousands of years ago.
Oddly, there are no specific set qualities that are universally identifiable for making the ingroup-outgroup distinctions. Having a different skin color might trigger an outgroup assignment, or it might not. Fans who cheer for the same sport team may have no other qualities in common but could still regard their fellow fans as ingroup members.
Creating an ingroup of all people, broadening perspective to include all of humanity as being equally valid in their desire for happiness, their desire to be free from suffering, is the first step towards removing the divisions we've created.
Until we are able to move beyond tribalism and exclusivity, we will continue to bare the fruits of rage and destruction that get created by this faulty logic. The time has come to break away from the narratives we've been taught, to discover a new way of perceiving our shared reality.