As we all know, WW2 started when the German Reich declared war on Poland, and in return France and the UK declared war on the German Reich.

But interestingly, when the Soviet Union invaded Poland from the East, the Allies did not bat an eye and did not declare war on the Soviets, what is the reason for that?

Was it because they couldn't afford a huge scale war with the Soviet Union, or was there an even bigger reason for that?