Why haven't France and Britain declared war on USSR in 1939 after they also invaded Poland?

by [deleted]

Greetings,

I was listening to a lecture on the topic of Post WW1 Europe and I asked professor this question and he couldn't answer this aside with some very unsatisfactory political claims of the time.

Many thanks in advance.

Georgy_K_Zhukov

This old answer should be of interest.