Tribes of neolithic humans occasionally attacked and killed each other, but it was about something real, like the survival of their families. Civilization has given us kings and armaments manufacturers, who have us kill and die over bullshit like "God" and "freedom".
While the military-industrial complex is a thing, most armed conflict can be abstracted as resource exploitation or contention expressed as territorialism. Religion and ideology is just a convenient cassus belli.
As the Lesser Bonapartes podcast puts it, "The in-group always has a problem with... those people."
Should just be one "s": https://en.wikipedia.org/wiki/Casus_belli
This might be true, but invariably it is massive excessive exploitation by the ruling class, rather than e.g. "we'd like to hunt in that creek bottom so we can eat some meat."