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.