What causes modern war to start? ‘Causation’ looks at the reasons for groups going to war with each other and analyses the inter-linked roles of nationalism, the state, empire, religion, and economics. A proper understanding of modern war will relate it to the forces of society and politics. There is much more than mere military involved in war today. What is the relationship between nationalism and war? The inherent and instrumental aspects of the appeal of nationalism help to explain why resorting to the awfulness of war seems reasonable to many people. Interwoven with nationalism, in many cases, is the role of the state. How important are religion and emotion to the causation of modern war?