This thesis explores the wartime experience of the League of Nations. It analyses the League’s ability to serve as a touchstone for international political, economic and social cooperation in a period of intense crisis for liberal internationalism. It demonstrates that the League’s political identity retained a relevance to a world at war, despite the failure of its diplomatic role. The thesis chronicles the efforts of League officials and of member states as they strove to maintain, in the League’s international civil service, a nucleus of liberal idealism in contradistinction to fascist expansionism. It determines the impact of geo-political factors on the integrity of the League apparatus and documents how the League’s ideological baggage determined its wartime social and economic work. The League did not remain a static entity in its final years and this work highlights the adaptation of League officials to an evolving political landscape with the League’s wartime experience providing a bridge between pre-war internationalism and its post-war variant. The successes and failures of the League’s political and technical organs were a reflection of the course of international affairs with its wartime history serving as a barometer of the diminished Eurocentrism and rising Atlanticism of international cooperation. This period was emblematic of the challenges of internationalism with the League’s international civil service splintering under the weight of internal and external pressures. The League’s wartime experience also underscored the reality that internationalism was a contested concept. The League’s brand of internationalism, with its aim of universalising the values of liberal democracy, was increasingly out-of-step with a war-weary preoccupation with security. League officials fought to preserve technocratic unity between the old organisation and the U.N.O. within an international order increasingly dominated by the two emerging superpowers; neither of which enjoyed a straightforward relationship with the League of Nations.