I have little knowledge about the truth here but I'd heard that this sort of warring stuff only ramped up after agriculture became common, no? Why kill other tribes' members unless they have a hoard of food? I suppose "to reduce competition" is one answer, but if everyone can hunt/gather for enough food, the motivation seems reduced.
OTOH if I can ignore preparation for winter and just raid your village for grain stores, the calculus changes.
The usual tribal conflict pattern is to seize territory as a hunting ground. Note that for nomadic tribes, there is no village and therefore no fixed location to raid.
E.g. the Comanches had a longstanding war* with the Apaches not because they wanted to seize each other's grain stores, but rather because they wanted control of the best buffalo hunting grounds.
* Not a war in the sense that we'd call it today. There were e.g. no organized charges, battle lines etc. Maybe "long slow series of irregular cavalry skirmishes" is more accurate than "war."
I am not an anthropologist, but I believe that's a popular misconception. Many technologically primitive tribal cultures have low absolute numbers of violent deaths, but they also typically have very low populations compared to agricultural societies. Their per capita casualties from intra- and inter-tribal violence are quite high.
Humans are Klingons. We in the developed world are currently living in a very strange universe unlike any before it in the history of humankind. Previous to the peaceful, violence-free lifestyle most people enjoy in the West everyone was pressed into sustaining the human war machine. American Indians had a long history of inter-tribal warfare to rival any European combat. It's just what humans do. Our apex societal violence is force-multiplied by a small group of people operating complex technological machinery, even though everyone else is still pressed into service and driven to exhaustion for industrial productivity.
> Why kill other tribes' members unless they have a hoard of food? I suppose "to reduce competition" is one answer
Yes, and not just for food and similar non-human resources. Reducing competition for mates is a common reason; even well past hunter-gatherer society, killing the men to take the women as mates remained a thing.
Tribal warring is common even in apes. Territorialism is especially strong. There are also plenty of things to war over besides food, such as space, shelter, mates, glory, gods, historical feuds, etc. Remember if we don't sacrifice one human before nightfall, the sun won't rise tomorrow.
Keeley says peaceful societies are an exception. About 90-95% of known societies engage in war. Those that did not are almost universally either isolated nomadic groups (for whom flight is an option), groups of defeated refugees, or small enclaves under the protection of a larger modern state. The attrition rate of numerous close-quarter clashes, which characterize warfare in tribal warrior society, produces casualty rates of up to 60%, compared to 1% of the combatants as is typical in modern warfare.
The Hobbsian trap: kill them before they kill you.
Minor motivations and proximity are enough to set it off. And thereare plenty of motivations anyway: fighting over good hunting areas, water, gathering areas, and women.
OTOH if I can ignore preparation for winter and just raid your village for grain stores, the calculus changes.