Some clues :
Douglas P. Fry : Pacified Past
Azar Gat : Warfare as an Ancient Feature
Robert L. Carneiro : Complexity and State Formation
Was the 20th century one of the most violent in human history ? (with two world wars and numerous other conflicts) ?
i also like the documentary series : “The Ascent of man” from the BBC in 1973 by Jacob Bronowski.
As a species we have always made war on each other. Our ancestal species also most likely did. Our closest living genus also frequently raids other tribes.
It is a part of who we are. However, like xenophobia it is something we can work to minimise with the goal to eradicating one day.