Book section: This text presents the hypothesis at the origin of this book, namely the crucial character of the Great War in the economic and social history of Europe in the 20th century, in comparison, in particular, with the Great Depression or the Second World War. It discusses the reasons for asking this question, both in terms of the evolution of the historiography of the Great War, and the place of this unprecedented conflict in the "spontaneous" representations of the 20th century (and thus among possible points of comparison for contemporary crises), particularly in the economic world, with the consequences this may have for economic policy choices made on a national or international scale. It presents the shocks resulting from the Great War: demographic and health shocks, rural and urban destruction, environmental damage, population movements, monetary and financial crises, instability of public finances, reduced trade, new fragilities of peace, the Bolshevik revolution. On the other hand, he considers several major areas in which the Great War was part of a longer movement, such as the rise of technosciences and social protection, transformations in business and work, the expanding role of States, and changes in international relations with the rise of national liberation movements in colonial empires and the establishment of international organizations. All in all, it suggests that the Great War may have been a unique turning point, from which the various local, national and international players drew the final lessons after 1945 and up to 1973, contributing to a new phase of globalization and proposing new answers to the nagging question of inequality.