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People are primates. They share a lot of DNA with primates and a lot of human behavior is straight-out monkey behavior. They divide themselves up into tribes, they compete for rank within those tribes and they get into fights with primates from other tribes.
If things are going badly in your tribe, anyone above you in the pecking order will try to look good by dumping on someone lower. You can make yourself look good by dumping on someone even lower than you.
If the tribe is threatened, the higher-ups might start looking for someone to blame. that'll be you, unless you can convince them the tribe's problems come from outsiders. that's usually where war comes from.