Chapter 9 deals specifically with the way in which the war affected the notions and practices of citizenship. It does so by switching the attention from enemy aliens to citizens of enemy origins. The chapter concentrates firstly on an analysis and discussion of the naturalization policies adopted by the various belligerent countries during the First World War. It then examines the spread of denaturalization statutes across Europe and the emergence of statelessness, concentrating both on state attitudes and public opinion and on the impact of naturalization and denaturalization policies on people of enemy origin. It also looks past the end of the war at the impact of those policies on interwar developments in inclusion and exclusion. The war played a crucial role in stabilizing differences between citizens and aliens and making them starker. It also imposed markers of identity (nationality, language, religion, ethnicity, “race”) on people, often regardless of their will or choice. In the name of military necessity and national security, authorities were willing to investigate origins and parentage or kinship, religion, language and all the markers that could indicate disloyalty to the nation in arms, thus implementing rigid notions of citizenship/subjecthood.