This chapter offers an historical grounding in interwar international relations. It tracks and analyses the progress of international relations in the period between World War I (1914–18) and World War II (1939–45), both of which are rightly seen as two major and formative conflicts in international history and indeed for the study of International Relations. It is sometimes assumed that the two World Wars were primarily European affairs, at least in their origins, and reflected the persistence of European predominance in a fast-changing world. Yet these were truly global and globalising wars, as reflected in their causes, courses and consequences, the technologies they employed and the ideas they helped generate. The period in between the wars was a turbulent and unstable one. It foreshadowed European decline and witnessed the rise of the United States, the challenge of the Soviet Union and the Far East and, more gradually, of peoples around the world subject to imperial rule – in short, the interwar period provided the foundations for the international system that developed over the following decades. Many of its contours are still visible today.