Relating German Generals’ opposition to Hitler’s provocations, and the Conference of Munich:
But when at 4 PM on that very same day, Neville Chamberlain announced he was flying to Munich, the Generals stood dawn in awe of their inspired Führer and his skill at bluffing.
Officially the USSR had not received the ultimatum that caused the declaration of war on Germany. It’s a good question though; as many in France and Britain despised the Soviets almost as much as Hitler. The nearest we actually came to war with them came in the Winter War when Stalin invaded Finland; the French were particularly keen to send troops to aid the Finns.
Churchill in The Second World War writes that the possibility of hostilities with the Soviets was not an impossibility;
“And what of the supreme, measureless ordeal in which we were again irrevocably plunged? Poland in its agony; France but a pale reflection of her former warlike ardour; the Russian Colossus no longer an ally, not even neutral, possibly to become a foe.”