Some 80 years ago, on Aug. 23, 1939, Nazi Germany and the Soviet Union signed the Molotov-Ribbentrop Pact, formally known as the "Treaty of non-aggression between Germany and the Union of Soviet Socialist Republics." The world was shocked — and terrified — by the agreement. Western democracies of the 1930s had counted on the huge resources of Communist Russia, and its hostility to the Nazis, to serve as a brake on Adolf Hitler's Western ambitions. Great Britain and the other Western European democracies had assumed that the Nazis would never invade them as long as a hostile Soviet Union threatened...