The other day in some comment box thread somewhere, I saw someone demand to know why it was that England and France declared war on Nazi Germany after its invasion of Poland, but not on the USSR, which (as agreed in the Molotov-Ribbentrop Pact) invaded Poland as well (plus anexxed the Baltic states and attacked Finland).
The person asking the question had an answer in mind. He felt that England and France must not have cared at all about the freedom of Poland or fighting authoritarianism, but rather cared about preserving their colonial empires -- to which Germany was a threat but Russia was not. I tend to find this explanation a little overly cynical. But the question itself remains interesting.
Looking into it more deeply, the first answer seems to be one of timing. Germany invaded Poland on Sept 1st, 1939. Britain and France declared war on Sept 3rd. German troops reached and attacked Warsaw on Sept 9th, and the largest battle of the campaign was fought along the Bzura River from Sept 9-19. On Sept 10, the Polish commander in chief, Marshal Edward Rydz-Śmigły, ordered a general retreat from the country through neutral Romania. (The Polish army made its way to France and thence to England. It remained the third largest active Allied army until the entrance of the US into the war.) Russia invaded on Sept 17th.
So one reason may well be that since the Russian invasion cam so late, it wasn't seen as being as much a direct violation of treaty as the German invasion.
Clearly, Allied sympathies ran strongly against the Soviets, and several thousand Brits and Canadians volunteered to go fight the Russians in Finland when Russia attacked her in November of 1939. However, since Germany is in between Russia and Western Europe, the Allies may already have fostered hopes of eventually winning Russia over to fight against the Nazi threat.
Also, having just read Antony Beevors books on the Eastern Front in WW2, I kind of suspect that in 1939 it didn't occur to Britain and France that Stalin could manage to be as much of a threat to the rest of Europe as Hitler was. The Russians essentially walked into Poland, and they were able to take the Baltic states without any fighting. They were badly mauled by tiny Finland. And after years of Stalin's purges and the general poor management for which the USSR is so justly famous the Russian army was in simply terrible shape in 1939-1941. It seems to me like it was only the invasion by Germany (and the immanent threat to Stalin's life and power) that forced the Soviet Union to lay off its self-cannibalism and build a military machine capable of destroying Germany and taking and keeping the Eastern Bloc. The resources were certainly always there, but without that incentive, I don't think that the Russian military would have been built into the machine that held half of Europe captive for forty years.
Thus, in a certain sense, we can lay the sufferings of Eastern Europe under Soviet rule at Hitler's feet. For all his hatred of Communism, it probably never could have conquered Europe without his unwitting help -- though that would still be little consolation for the tens of millions of Stalin's domestic victims.