This is the story of how the world's two greatest (or worst) totalitarian powers reached an agreement to carve Europe between them. Hitler was obsessed with avoiding a two-front war as happened to Germany in WWI. This time, though, the situation was reversed. He sought to knock out the Western powers first before turning to the East. France and England were the major worries at the moment.
Stalin also wanted a free hand as he sought to restore the USSR's border's to pre-Revolution range. This naturally included a division of Poland and the absorption of part of Eastern Europe. One is struck at the gall of these powers sitting at a map and drawing lines, dividing the civilized world into spheres of influece, knowing all the while that in the end, they will have to fight.
The authors record the pre-talks, the feelers, the struggles of the Western powers to stop this deal at any cost. But Hitler was determined to press ahead and secure at least half of his border. There are several mini-tales included that were affected by the treaty - the tragic dismemberment of Poland, the Russian rape of Finland, the beginning of a pattern embraced by both powers and continued by the USSR after the war: The absurd claim that a government would ask either power to invade its territory in order to crush "warmongers".
Both nations shocked their supporters - Germans were puzzled as to why such an agreement was needed with its arch-enemy. Leftists worldwide were struck dumb as their hero, Stalin, smiled and signed on the dotted line. But there was nothing to fear. As the fighting wore on and England refused to bow, Hitler planned the final punch - knock the USSR out of the war and England would be forced to sue for peace. It was almost a success but the supply lines and huge area became a quagmire and the lost retreat was in place. The treaty had served its purpose and like most treaties signed with totalitarian powers it remained in force as long as it was needed.