The liberal international order established at the Paris Peace Conference was overthrown between 1933 and 1939. This opened the way for Nazi Germany and Fascist Italy to launch wars of conquest aimed at creating empires in Europe and the Mediterranean. This chapter considers whether the outbreak of war in September 1939 should be understood as a failure of European diplomacy. Peace is considered to be the ultimate aim of all diplomatic practice, even in wartime. One of the most important legacies of the First World War was the introduction of new international norms and new standards of international legitimacy. During the 1930s, professional diplomats in Britain and France failed to provide clear and effective policy guidance to their respective governments. The foreign policies of both states were slow to adapt to the changed international circumstances of the 1930s.