Why did UK and France not declare war on USSR when they invaded

Stalin wouldn't have invaded Poland on his own.

>>896270
>Why England and France were not suicidal

I don't know

because like you rightly say, their treaty specified the german threat, but unlike you claim later on, russia was simply not just as much of a threat at the time - germany was fresh on the back of occupying/annexing/breaking treaties

however, it was soon perceived as such and indeed during the winter war the western allies were surprisingly close to actively and militarily supporting finland against the soviet union

>>896293

This. Even as it happened they waited for Germany to destroy most of the Polish military before rolling in.

>>896270
Because muh cousin the tsar

File: george_v_and_nicholas_ii.jpg (141KB, 800x607px) Image search: [Google]

141KB, 800x607px

>>896506
This. You can even see the resemblance.