At first glance Molotov-Ribbentrop looks ASB, but Stalin needed it with much of his army still in the East, and Hitler wasn't averse to making promises he had no intention of keeping.
But, suppose Stalin was convinced that Britain and France were determined to defend Poland, and had made it clear to them privately that he would not intervene, hoping that a Nazi/Wallies war will prove damaging to both? (He's not going to want, for example a permanent RN base in Szczein or Danzig).
Hitler invades Poland as OTL. What is the best Wallies strategy? A second front in the Ruhr?