During the first half of the twentieth century, world politics was reshaped in pursuit of a new international order. The ideological foundations of the new diplomacy (and its fate during the interwar period) are well known. This book instead examines the practices of internationalism and diplomacy from the First Hague Conference of 1899 to the aftermath of the Second World War. By focusing on these practices, such as disarmament regimes or public diplomacy, and their use as instruments to build international order(s), it emphasises the constructed, contested, and experimental character of what subsequently became a standard repertoire of international politics. Essays from a range of interdisciplinary scholars address well-established principles such as self-determination, and also less prominent practices such as small arms control or parliamentary inquiry. The book makes a major contribution to the growing historiography on twentieth-century internationalism.