In 1938 the great powers met in Munich to determine the fate of the Sudetenland, a territory belonging to Czechoslovakia. Even though the Soviet Union was a great power, and had an obvious interest in the region, it was not invited to the conference.
As we all know, Britain and France caved in to Hitler. Stalin learned a lesson from all that. The British and French were weak. The initiative belonged to Germany.
So Stalin allied with Hitler, not out of any common cause, but because the West would not stand up to Germany.
(Again, this is a Devil's Advocate viewpoint...just something to think about.)
Stalin actually were actively looking for alliance with both British and France against Germany just prior to Molotov-Ribbentrop which is itself anything but alliance. It was actually a non-aggression pact, not alliance.
.....the Moscow-controlled Communist International, and its sidekick, the Communist Party of Germany, made Hitler’s rise to power possible, if not indeed inevitable, by tarring the German Social Democrats as “social fascists” who threatened to split the proletariat and were, thus, a greater evil than the Nazis.
I recommend Stephen Koch's book on Willi Münzenberg, Stalin's man in the West: Double Lives: Stalin, Willi Munzenberg and the Seduction of the Intellectuals
The book relies on several memoirs and testimonies from agents defecting from the Soviet "Apparat" (Soviet Secret Services). But one can also read Arthur Koestler's autobiographies "Arrow In the Blue" and "The Invisible Writing" to learn how closely Stalin and Hitler cooperated, and how that facilitated Hitler's power grab in Germany.
Then we have the collaboration between the German Army and the Soviets that started already in the 1920's, and without which the Wehrmacht never had been able to grow so mighty.
The point you bring up (I know you are playing the Devil's advocate) is the typical left-wing defence of Stalin (See Wikipedia) and the 1939 deal between Nazi-Germany and Soviet-Russia:
Joseph Stalin was also upset by the results of the Munich conference. The Soviets, who had a mutual military assistance treaty with Czechoslovakia, felt betrayed by France, who also had a mutual military assistance treaty with Czechoslovakia. The British and French, however, mostly used the Soviets as a threat to dangle over the Germans. Stalin concluded that the West had actively colluded with Hitler to hand over a Central European country to the Nazis, causing concern that they might do the same to the Soviet Union in the future, allowing the partition of the USSR between the western powers and the fascist Axis. This belief led the Soviet Union to reorient its foreign policy towards a rapprochement with Germany, which eventually led to the signing of the Molotov-Ribbentrop Pact in 1939.
There is definitely a point there - but only if one disregards the earlier and ongoing German and Soviet collaboration and the diplomatic play by the Soviets during the years preceding the Munich conference.
The Soviet Union and France had both defence treaties with Czechoslovakia. The official Soviet history (edited by Gromyko and Ponomaryov) unsurprisingly states that the USSR all the time was prepared to carry out its treaty obligations.
However, the German diplomats who reported to the German FO were totally convinced that the Soviet Union did not intend to assist Czechoslovakia in September 1938. It should also be noted that the Soviets offered Germany a non-aggression pact already in 1935.
One should also note the little remembered affair of the Czechoslovakian mobilization in May 1938. Not only did it harden Hitler's demands, but the reasons for it also turned out to have been erroneous; the Nazis were not mobilizing against Czechoslovakia at that time.
The Czech historian Igor Lukes writes about this episode here:
Stalin's Diplomatic Maneuvres During the 1938 Czechoslovak Crisis
This essay is well worth a read - it gives some important answers to your question, and it shows the Soviet Apparat involved in high politics, and not unlike the foreplay to the Six-Day War in 1967, it appears that the Soviets played a client nation into committing a premature military action; in May 1938 Czechoslovakia, in May 1967 Nasser's Egypt.
But that's another story....
USSR didn’t even have a boundary with CS. Poland was in between, and Poland, in the most astonishing short-sightedness, did not support CS but instead joined in feasting on the remains. Poland, reasonably enough, was about as suspicious of Stalin as Hitler. They had, after all, fought a war with USSR in the aftermath of WWI in which Warsaw was very nearly taken.
I wonder who the Poles thought would be next on the menu.
Stalin knew the Western allies were weak when they did nothing to stop communism in Spain in 1936, and they let Germany re-arm (with Soviet help) throughout the late 30s. In 1939 they divided Poland between them (though for some reason the western allies didn’t declare war on the USSR - only Germany); they also stood by while Stalin invaded the Baltic states and part of Finland.
While Americans have proud moments in our history, the years leading up to WWII are not among them.