William L. Shirer, The Rise and Fall of the Third Reich
From the moment when Molotov became Foreign Commissar he pursued the policy of an arrangement with Germany at the expense of Poland. It was not very long before the French became aware of this. There is a remarkable dispatch by the French Ambassador in Berlin, dated May 7, published in the French Yellow Book, which states that on his secret information he was sure that a Fourth Partition of Poland was to be the basis of the German-Russian rapprochement. "Since the month of May," writes M. Daladier in April 1946, "the U.S.S.R. had conducted two negotiations, one with France, the other with Germany. She appeared to prefer to partition rather than to defend Poland. Such was the immediate cause of the Second World War." But there were other causes too.
Winston S. Churchill, The Gathering Storm
5/7/39 update at #16. Case White and wooing Stalin.