Try to fix the confusion - Considering Britain had a military alliance to protect Poland in 1939. All of you guys know that they declared war on Germany when the Germans invaded Poland but not the Soviet Union when they invaded two weeks later.
By the time the Soviet Union invaded Poland, both the western powers of Britain and France knew that Germany was the real threat. They did not want to risk driving the soviets closer to Germany. Both countries wanted to pull Russia into alliance.
If Britain and France were going to declare war on the soviets it would have most likely been during the finish winter war. Not to mention if they did declare war on Russian they really didn't have a way to attack Russia and they needed to focus all their resources into fighting Germany.
Sources: The third reich at war John Keegan's "the Second World War"
The Soviet Union was expelled from the League of Nations for their aggression towards the Baltic states (which the took over) and Finland. This was probably as far as the West would go, however, as noted below they simply didn't have the resources or ability to confront both the USSR and Germany.
Just adding to what others have said, but before the war broke out (and indeed during the early stages) it was hoped that Germany and the USSR would fight, them being the greatest regional powers. A declaration of war on the USSR and Germany for the same reason would only have driven them closer together. They already considered themselves the two isolated powers of the world that nobody would trade with or acknowledge; war with them both would only have confirmed this in their eyes.