Margaret MacMillan, the historian whose previous books examined the Paris Peace Conference and the origins of World War I, considers war as a persistent aspect of human experience that demands serious examination rather than dismissal. MacMillan explores how warfare has shaped human societies, from prehistoric skirmishes through ancient empires to modern total war. She examines why human beings fight, finding explanations in evolutionary psychology, social organization, culture, and ideology. The book considers how war has changed over time, from the limited aristocratic conflicts of the eighteenth century through the industrial slaughter of the world wars to contemporary asymmetric and cyber warfare. MacMillan discusses the experiences of those who fight, drawing on memoirs, literature, and psychological research to understand how combat affects individuals. She examines the relationship between civilians and military, how societies commemorate and remember wars, and the persistent fantasy of abolishing war entirely. MacMillan writes neither as a pacifist who condemns all war nor as a militarist who celebrates it, but as a historian who believes understanding war is essential to understanding human civilization. Readers will find here a thoughtful and wide-ranging exploration of a topic that resists easy conclusions but demands engagement.