Why do humans go to war? Have we been waging war ever since we first existed as a species? Is a propensity to wage war part of what it is to be human, or more a result of the evolution of human society? And has there been a decline in war-making over time - or is this just a pious hope?Azar Gat here draws together insights from evolutionary theory, anthropology, history, historical sociology, and political science to address these fundamental questions about the history of our species - the answers to which also have big implications for our species' future survival.The book reveals that theories regarding the recent decline of war, such as the "democratic peace" and "capitalist peace", capture merely elements of a broader Modernization Peace that has been growing since the onset of the industrial age in the early 19th century.