June 2019 marks the 100th anniversary of the Treaty of Versailles and the foundation of the League of Nations, which created new legal techniques and shaped the development of international law. When considering how international cooperation and international law increasingly are being challenged in our contemporary world, it is right about time to revisit a cornerstone of their historical foundations, namely to explore how the Treaty of Versailles and the League of Nation Systems (i.e. the League of Nations, the Permanent Court of International Justice and the International Labour Organisation) influenced the development of international law from 1919-1945.