President Trump recently praised the Soviets as a "great ally" in the second World War. And although it's true that the USSR faced down the full force of the Wermacht and suffered tremendous losses, it seems like it also shares some responsibility for the start of the war.
The Molotov-Ribbentrop pact emboldened the Nazi invasion of Poland and allowed the Soviet annexation of Latvia, Lithuania, and Estonia. Annexing Bessarabia, Bukovina, and the Hertza region from Romania and invading Finland helped push these countries to the Axis side providing hundreds of thousands of men to the Eastern front as well as essential resources--the Ploiesti oil fields provided as much as 29% of the German oil supply in 1940.
What is the consensus of current historians on the Soviet role in the start and expansion of the war: did these actions (and others) enable the initial Nazi expansionism of 1939 to evolve into a World War? Or was it inevitable that the war would engulf the continent and Stalin was shrewdly playing a bad hand?
No doubt Ribbentrop-Molotov played a huge role in emboldening Nazi aggression in the early years of the war, but its hard not to sympathize with Stalin's predicament.
First of all, its important to keep in mind that the R-M Pact was signed in August of 1939, less than two weeks from the outbreak of the war. This comes a full year after disastrous appeasement politics have all but condoned Germany's annexation of both Austria and Czechoslovakia, and three years after Hitler's militarization of the Rhineland. By 1939, Germany had been violating the conditions of Versailles for over six years without any true repercussions. Russia's sheer manpower and territory lent it significant influence in European politics, but it remained relatively withdrawn from the affairs of the continent during the 1930s.
On the occasions that Stalin did become involved in these matters, it was usually in opposition to Germany. During the Spanish Civil War of 1936 Russia sent military aid to Spanish Republicans, supporting their efforts against the German backed fascists. The anti-communist vitriol spewing from the Nazi propaganda machine did nothing to warm relations over this span. Regardless of Soviet sentiment towards Germany, there was little that Stalin could have done during the Appeasement years to stem the rising tide of German militarism and expansionism. The Communist nation was a nascent industrial power, but it had yet to truly leverage the potential of its vast population. Economic developments under Stalin's second Five Year Plan were beginning to take hold but Russian production was not anywhere near adequate war time capacity until 1942. Stalin was aware of his country's growth trajectory, and realized that he could not afford to go to war with a modern military power as early as 1939.
If ever there was a chance for this to turn out differently, it died with the British diplomacy of 1938 and 1939. Stalin felt spurned by Western Europe after the agreement at the Munich Agreement, allowing Germany to expand onto Russia's Eastern frontier into a Communist-International member state. Nonetheless, so precarious was Stalin's position in the summer of 1939 that he continued to entertain aspiration for an alliance with Britain. There were talks, but they sputtered and died mere weeks before the R-M Pact was signed. If the USSR were going to take a strong position against Hitler it was going to be with British backing or not at all.
Ultimately Stalin was just as fearful of the Nazi threat as any of the future allied powers. The Soviet Union had very little to gain and everything to lose from the encroachment of a military-industrial giant into Eastern Europe. However, faced with the reality of that happening they had to make a choice. Without a powerful ally or prepared military, the only logical choice was to align with Hitler in the name of self preservation.