An award-winning historian plumbs the depths of Hitler and Stalin's vicious regimes, and shows the extent to which they brutalized the world around them.Two 20th century tyrants stand apart from all the rest in terms of their ruthlessness and the degree to which they changed the world around them. Briefly allies during World War II, Adolph Hitler and Josef Stalin then tried to exterminate each other in sweeping campaigns unlike anything the modern world had ever seen, affecti