Within the history of the inter-war years, the Soviet-German relationship stands out as being decisive not only for Europe, but the world as a whole. Before Molotov and Ribbentrop had heard of each other, the German and Soviet armies had established a functional working relationship which would yield the tools later used in the Second World War. This study traces the evolution of the relationship between the outsider states of the international system from its earliest moments in the chaotic months following the First World War, to the establishment of formal relations in 1922, when the diplomatic umbrella could more safely shelter the growing collaboration. Making Common Cause is the first book-length study of this topic to be based upon both German and previously underutilized or unavailable Russian archival sources, and thus provides an important complement to the historiography of inter-war Europe.