Drawing on archives in five countries, Johnson delves into the fascinating secret military cooperation between Germany and the Soviet Union in the interwar period. After the end of World War I, a defeated and disarmed Germany sought ways to rearm despite the severe restrictions imposed by the Treaty of Versailles. Meanwhile, most major powers refused to recognize the new Bolshevik government in Russia, which was in dire need of investment and foreign assistance to build up its armed forces. The two countries’ pariah status drew them together. The Soviet Union provided a place—beyond the reach of Allied inspectors—for the research, development, and testing of German combat aircraft, tanks, and chemical weapons. Cooperation with Germany played a crucial role in the development of the Soviet military industry and Red Army cadres. Thousands of Soviet military officers trained alongside their German counterparts. Later on, Hitler’s massive rearmament was enabled in part by the military capabilities that Germany had developed in the Soviet Union before the Nazis came to power. Hitler ended German-Soviet military cooperation in 1933, soon after he became chancellor. But in 1939, the two countries grew close again: they signed the Treaty of Nonaggression, renewed their military ties, and agreed to partition eastern Europe between them. Their extensive trade partnership continued until Germany invaded the Soviet Union in 1941.