This is the last post I will make, so as to not derail the thread, but from the perspective of the Soviet Union, France and Britain sold out to the Nazis at Munich. Czechoslovakia had a defensive agreement with both France
and the Soviet Union; the agreements were intended to prevent war in central Europe by being a) defensive in nature, and b) ensuring overwhelming military force would be sent against Germany.
During the Munich talks, Stalin had announced his willingness to defend Czechoslovakia from Germany, even if France wouldn't, provided his decision got Franco-British blessing and the French used their influence to persuade Poland to permit transit of Soviet forces into Czechoslovakia. Poland at this time was in the on phase of its on-again/off-again relation with Nazi Germany, and was even mobilizing forces to grab territory from Czechoslovakia. With Nazi Germany actively courting Poland by offering it land from Czechoslovakia that Poland believed it had claim to, Poland was not going to permit Soviet troops transit to defend Czechoslovakia. I believe (though I can't remember the source) that Britain and France were particularly afraid of increased Soviet influence in Europe if they allowed the Soviet Union to defend Czechoslovakia.
When Czechoslovakia was partitioned, from the Soviet point of view it was hard to escape the viewpoint that treaties with France and Britain were worthless, and that France/Britain preferred to sell out their allies to Nazis than let the Soviet Union into the 'European Club'. In that context, the Soviet policy toward Europe in the next few years makes a lot more sense. The Soviet Union made a big diplomatic effort to normalize relations with France and Britain and be accepted as part of the European Club, only to be kicked out so France could ignore its treaty obligations to the Czechs and Nazi appeasement.
https://en.wikipedia.org/wiki/Munich_Agreement
https://en.wikipedia.org/wiki/Franco-Soviet_Treaty_of_Mutual_Assistance
https://en.wikipedia.org/wiki/Czech_Republic–Russia_relations#Background