The histories of modern war and childhood were the result of competing urgencies. According to ideals of childhood widely accepted throughout the world by 1900, children should have been protected, even hidden, from conflict and danger. Yet at a time when modern ways of childhood became increasingly possible for economic, social, and political reasons, it became less possible to fully protect them in the face of massive industrialized warfare driven by geopolitical rivalries and expansionist policies. Taking a global perspective, the chapters in this book examine a wide range of experiences and places. In addition to showing how the engagement of children and youth with war differed according to geography, technology, class, age, race, gender, and the nature of the state, they reveal how children acquired agency during the twentieth century’s greatest conflicts.