What is the ultimate source of war?

Possessiveness—the passion to have and to hold—is the ultimate source of war, and the foundation of all the ills from which the political world is suffering

Excerpt from Political Ideals by Bertrand Russell (1917)
Woman planing propeller in factory, by unknown military photographer during WWI in England. Source: UK Imperial War Museum’s First World War Women’s War work Collection. Licence: Wikimedia Commons