International relations scholars and historians have long debated the reasons for Siam's maintenance of formal political independence during the colonial period. Territorial concessions and intra-great-power agreements are recognised as important, but attention is beginning to be paid to the solidarist and normative aspects of Siam's diplomacy. I argue that Siam's stance in World War I, first as a neutral nation, and then as a belligerent, allowed it to demonstrate mastery of European conventions of war. In line with the predictions of the English School theory of international relations, this increased Siam's status in international society. This allowed Thailand to participate in the Paris Peace Conference, providing Thailand with a better position for its campaign to rid itself of extraterritorial legal provisions and unequal treaties. Thailand's rules-based and normative participation in World War I also offered the maximum opportunity to benefit from the advent of the new liberalism in international affairs that was then emerging, especially through the ideas of United States President Woodrow Wilson. I argue that Siam's use of its military force as a passport of sovereignty, a membership card of the family of nations, has had important long-term implications for its understanding of the role of military force in its international relations.