Richard Overly's Russia's War discusses the diplomatic attempts by the USSR to create a military alliance with British and French just after Czechoslovakia, to more effectively deter Germany with a giant ground force staged on their western and eastern borders, but UK and French representatives did not have any authority to make decisions, only talk, were uncommitted, so Stalin bailed and then took up German offer for economic talks which led to non aggression talks.There is more to it than just that. One of my books I read in the last 3-4 months mentions it in detail, but I can't remember which one. There is more info on the Wikipedia page with sources, but I'll try to figure out which book of mine detailed it.
Bottom of Munich Agreement section
UK especially, and some degree France but nothing like the Briitsh, were attempting to use Poland to curtail not only German expansion East but Soviet expansion West.
The non-aggression pact wasn't designed to allow Hitler to invade Poland, there was zero about them at the time, it was to buy some time where Germany and Soviets didn't need to worry about one another. Poland came later when Hitler quickly had the General Staff draw up plans for Case White only months before the invasion occurred, with only short notification (a few weeks, maybe a month, if memory serves) that the Soviet Union was aware, who gave their blessing only because Hitler bribed them with Eastern Poland.
Written in 1997, so before Putin came to power, so not influenced by Russian national apologists.
The current unclassified letter gives more details but its already historical fact it occurred.
Knowledge of and acceptance of this information should completely change the framework of this discussion, and it should be interesting to see how it goes from here.