Every group has the tendency to unite when faced with a threat.
It's an adaptive instinct to survive in environments when threats were more serious than today.
Today we unite when faced with existential threats, perceived threats, and even minor insults. We unite if someone removes our right to do something (or we think they will). We unite when one of our group (or a friend of the group) is attacked (or faces a perceived attack).
This unity has behavioral implications.
We are more confrontational towards outsiders, more exclusionary against fringe/new members, and try to replicate the behavior of the group's prototypical members (we all act the same).
There is only one exception, hopelessness. If we feel we're doomed, that the threat is too big or our own power/resources too small, we disband. We take whatever approach best enhances our own sense of success. We grab and run.
History's greatest leaders (and monsters) have all used the sense of threat to achieve their goals. It's a controversial, but valid method. A threat turned into a victory becomes a powerful unifier for any group.
If you're going to use this method, you need to achieve two goals.
First, create a threat that your members take seriously.
Second, always assure members that if they unite, share resources, each take on their fair sure, they will succeed. Never admit you might not succeed.
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