A cartoon that appeared in London’s Evening Standard in September 1939 shows Adolf Hitler and Joseph Stalin tipping their hats and bowing to each other. “The scum of the Earth, I believe?” says Hitler. “The bloody assassin of the workers, I presume?” Stalin replies. A month earlier, the two leaders had concluded a pact promising not to interfere in each other’s aggressive military campaigns and devised a secret plan to divvy up the lands between their countries. Moorhouse captures the essence of the wretched deal better than anyone has before. As they ripped Europe apart, Berlin and Moscow danced an awkward ballet, straining to preserve their compact while mutual mistrust mounted and their armies deported large segments of the local populations in captured territories. Moorhouse concludes by tracing with new detail the stages by which Hitler ultimately decided to invade the Soviet Union and, on the other side, Stalin’s bewildered efforts to both deny and prepare for the double-cross.