When I was in elementary school,1 there was a group of boys who would do a certain thing.
First, they would seek out a pair of close friends. The closer the better.
It is extraordinary how instinctively and swiftly tribalist boys encircle prey, covering all exits.
Then the boys in the circle would push the friends into each other repeatedly.
More boys would join the circle, for a total of perhaps twenty. They would yell, "Fight! Fight! Fight! Fight! Fight!"
Bystanders would join silently. They couldn't be distinguished from those cheering.
If the friends resisted, the pushing would continue harder. If one fought with a member of the circle, he would be hit and pushed at his friend again.
The friends would resist, but after many pushes over a long time, eventually one would think the other did not resist enough, or took inadequate precautions; or in the confusion, one would reflexively strike, which then looked deliberate.
The friends would fight.
Here is an old Star Trek clip. It lacks gravity and ignores complexities, but it makes a point.
What I'm saying is that we have powerful external enemies. Amid the flying fists, let's not forget their existence.
1 Lest the gang behavior seem rare: it can happen anywhere in the world, not only in ghettos as often portrayed (and not only by boys). In fact, the events in this story happened in a rather high-status place that was regarded as enlightened and not violent.