The fight against human trafficking, improvements in public health, or combating the international drug trade – these are some of the most pressing problems Thailand and the world are facing today. Remarkably, these topics were already high on the international political agenda over 80 years ago during colonial times, when the League of Nations, the United Nations’ predecessor, was created. This first in-depth study of Thai foreign relations in well over a decade traces how these and other policy areas brought Siam in contact with the League of Nations, after the kingdom had signed the Treaty of Versailles and became an original member of this first global body. Based on never before consulted primary sources in Thailand and Europe, the study unfolds the story of a unique relationship between the only independent country in Southeast Asia and the League during the inter-war period of 1920-1940. The book highlights both the importance of the League for the modernisation of Siam and the shaping of its foreign policies, as well as the intriguing role Siam played on the world stage in the early development of the multilateral political system we live in today.