The great anthropologists like Marvin Harris (as opposed to the intellectual pygmies we find in academe today) were unafraid to tackle the issue of war in primitive societies. In Harris's 'Cannibals and Kings' he devotes a chapter to the "Origin of War". Significantly, Harris found that anthropologists had found only a handful of societies that ostensibly did not make war. However, Harris found that those usually mentioned (Andaman Islanders, Shoshoni, Yaghan of Patagonia, Tasaday of the Philippines) were really refugee cultures. He notes that warfare is as old as time, and even the famous Peking Man had his skull smashed at the base, an indication of warfare.
Many of our Indian tribes didn't go to war over resources and hunting grounds. Very often the wars were fought for warrior's honor (bragging rights) and to capture the enemy's women for sex slaves and to make babies for YOUR TRIBE. Larger the tribe the stronger you are to conquer other tribes