Résumé : This thesis consists in a critical analysis of international law’s traditional historical narrative about the prohibition of the use of force. Most contemporary textbooks teach that this prohibition was a creation of the twentieth century, and that beforehand States were free to resort to armed force against each other unconstrained. Positive international law, the story goes, was ‘indifferent’ to the use of force – it did not prohibit it but did not authorize it either, which meant that, in practice, States could do as they pleased. ‘Reality’ as it stems from historical sources, however, appears much more complex. In fact, not only did the vast majority of nineteenth century authors claimed war and measures short of war to be strictly ring-fenced by international law, but it also seems that States quasi-systematically felt the urge to justify their actions when they employed force against another nation. Starting from the observation of this discrepancy and using tools of history, sociology, anthropology and social psychology, the present research seeks to understand the roots of the ‘indifference’-narrative and how it became the commonly accepted version of the history of the use of force in international.