When the tribe begins to outstrip its resources in the area someone has to go, either the restless or the less desired, willing or unwilling. Some contend that warfare began only after agriculture and settlement. They would like to believe in some Edenic goodness in earlier man that has somehow become corrupted with each advance of civilization. Conflict between bands or tribes seems to me to be an inherent consequence of man's success and expansion. War is just a succession of improvements on what man has been very good at for millions of years.
Most male animals compete with their peers for sexual dominance and men are no exception.
However, there is a unique cultural aspect to the human contest for genetic survival. Men will go to war and risk their personal survival and reproduction for that of the group. This strange behaviour against the individual's most basic self interest is probably bred in the bone. No doubt it served the genetic evolution of human beings well in the past but it does not appear to be constructive in the present. This country certainly has lots of diverse groups but some perception of the common good has generally been sufficient to overcome the strains.