In the vast literature about Stalin and Hitler during World War II, little is said about their being allies for twenty-two months. That is more than an odd chapter in the history of that war, and its meaning deserves more attention than it has received.

Two factors were involved in this neglect. One was that after Hitler chose to conquer Russia he did not succeed; Stalin emerged as one of the supreme victors of World War II. The other was the Western Powers’ relative lack of interest in Eastern Europe. Yet the war broke out in 1939 because of Eastern Europe, as a result of the British (and French) decision to oppose the German conquest of Poland. The political earthquake of the Nazi–Soviet Pact of August 23, 1939, nine days before the outbreak of war on September 1, did not deter Britain and France from declaring war on Germany upon its invasion of Poland. This is one of the few—very few—decisions in their favor at the time. That they were reluctant in the months that followed to wage war seriously against Germany is another story.

Three quarters of a century have now passed since 1939. A fair amount has been written about the Nazi–Soviet Pact since then, mostly by Eastern European writers and historians. The Devil’s Alliance is a good account by the British historian Roger Moorhouse of what the pact meant for Hitler and Stalin—and, worse, for its victims. Perhaps the book’s most valuable part deals with the immediate consequences of the pact in 1939. Before then, obviously and stridently, Nazism and communism were outright enemies. From the very beginning of his political rise Hitler described Judaism and communism as his principal enemies. Stalin, by that time, was less of an ideologue. Like Hitler, he was a nationalist; he had little interest in international communism.

In May 1939 Hitler recognized that in a German war with the West, Russia could be neutral. But to bring that about would not be simple; Russia would have to get something in return. During that summer a British-French delegation seeking some kind of a military alignment with Russia got nowhere. There were high German officials and diplomats, foremost among them Hitler’s foreign minister, Joachim von Ribbentrop, who were much inclined to a German–Russian deal. Meanwhile in Moscow Stalin had already moved: in early May he dismissed his foreign minister, the Jewish Maxim Litvinov, replacing him with Vyacheslav Molotov (who would remain his foreign minister during the war and well after). Hitler understood what that meant. In August Ribbentrop flew to Moscow to sign a Nazi–Soviet Non-Aggression Pact; a photograph of Ribbentrop with pen in hand shows Stalin visibly content in the background.…