This introductory chapter sketches the main argument of the book and the various areas of interest it touches upon: sovereignty, international law and laws of war, the dilemma security versus freedom, the attempt at humanizing the war conduct, the transformation of notions and practices of citizenship, rights, individual and human in particular, humanitarianism, national belonging, war and society. The introduction focuses in particular on whether there were consolidated customary practices and codified rules to deal with aliens of enemy nationality at the moment of the outbreak of the First World War, and if and how that conflict represented a watershed. It then presents the structure of the book.

This chapter opens the first part of the book that presents the background of the First World War. It deals with the emergence of the concept of “enemy alien” in the debate among international lawyers. Starting with the Law of Nations published by Emer de Vattel in 1758, it analyzes and discusses what the foundational texts of international law in the century-and-a-half preceding the First World War said on the rights of foreigners in peacetime and on the conduct toward these same foreigners when they became enemies in wartime. It then compares legal doctrines and practices analyzing the behavior of belligerents towards enemy aliens in a string of interstate wars that occurred between the end of the eighteenth century and 1865, namely the French Revolutionary Wars of 1792–1793, the Napoleonic Wars, the War of 1812 between Britain and the United States, the Crimean War and the American Civil War. The chapter follows the changes in the attitude toward enemy aliens that mass conscription and the post-French Revolution concept of citizenship and nationality triggered.