Since the 19th Century, armed reprisals escaped the efforts of regulation, despite at that time being a burning issue in international law. Why was this? Beginning with the law of reprisals in Medieval Times and its progressive obsolescence in Modern Times, this study demonstrates that the great Powers made a privilege out of this employment of force in peacetime and kept it in a legal grey zone. This enabled them to resort to armed reprisals against small States without incurring the consequences of a formal war. The work also explains the legal scholars’ hesitant attitude to clarify these armed reprisals and shows why the League of Nations failed to solve the problem.