France and Britain declared war on Germany after Germany's invasion of Poland, starting World War 2. Two weeks later, the Soviet Union also invaded Poland. I suppose declaring war on the USSR too was never a serious possibility, but were there other consequences for the Soviet Union? What did the public in France and Britain think about diving into war over the first invasion and ignoring the second?
There was concern but the French and the British decided to not intervene because their instinct and analyzation led them to believe that eventually the Germans and Soviets would turn on each other. They were proven right in 1941 when the Germans began Operation Barbarossa, and invaded the USSR.
The British reaction was of awareness and caution. When the Polish asked for assistance against the invasion, the British rejected for multiple reasons: the first being is that they were still reviewing their options in regards to Europe and the German invasion. The second reason was similar to the first; they prioritized the defeat of the Germans over anything else, even the USSR. The British were aware of how much the Soviets and Germans despised each other, even after both sides signed the Molotov-Ribbentrop Pact. Now the Poland was gone, there was no country or barrier in between both of the nations and nothing stopping one from invading the other. The belief was that eventually both nations would turn on each other and it was just a matter of time until that happened.
The only form of condemnation or expression of disapproval sent by the British or the French was when the British sent a letter to Stalin expressing their concern and disapproval.
The French response was nearly identical. They waited out and viewed their options with a sense of caution. The French however had a rather interesting belief; they would have a much better time in eventually dealing with the Germans because they assumed the Germans would never fully trust the Soviets and as a result they would keep most of their troops on the eastern border in Poland. They also made a more finalized and bold decision in comparison with the British, that Poland did not exist anymore, at all or in any form. However at the end of the day they too, like their British counterparts believed it was only a matter of time until the two sides went to war. As such they continued to host diplomatic relations with the Soviets with their new belief of what would happen in the future in mind.
The French were taken over by the Germans about a year later and the Germans invaded the Soviet Union two years later. The Soviets finalized the defeat of the Germans in the east as they entered Berlin in May, whilst France was liberated by the US, UK, Canada and other allied factions from the west.
Overall the reactions from both the British and French was caution, awareness and most importantly:
“They’ll turn on each other”. They were right.