The communists spent the decade prior trying to form an anti-Nazi coalition force, such as the Anglo-French-Soviet Alliance which was pitched by the communists and rejected by the British and French. The communists hated the Nazis from the beginning, as the Nazi party rose to prominence by killing communists and labor organizers, cemented bourgeois rule, and was violently racist and imperialist, while the communists opposed all of that.

When the many talks of alliances with the west all fell short, the Soviets reluctantly agreed to sign a non-agression pact, in order to delay the coming war that everyone knew was happening soon. Throughout the last decade, Britain, France, and other western countries had formed pacts with Nazi Germany, such as the Four-Power Pact, the German-French-Non-Agression Pact, and more. Molotov-Ribbentrop was unique among the non-agression pacts with Nazi Germany in that it was right on the eve of war, and was the first between the USSR and Nazi Germany. It was a last resort, when the west was content from the beginning with working alongside Hitler.

Harry Truman, in 1941 in front of the Senate, stated:

If we see that Germany is winning we ought to help Russia, and if Russia is winning we ought to help Germany, and that way let them kill as many as possible, although I don’t want to see Hitler victorious under any circumstances.

Not only that, but it was the Soviet Union that was responsible for 4/5ths of total Nazi deaths, and winning the war against the Nazis. The Soviet Union did not agree to invade Poland with the Nazis, it was about spheres of influence and red lines the Nazis should not cross in Poland. When the USSR went into Poland, it stayed mostly to areas Poland had invaded and annexed a few decades prior. Should the Soviets have let Poland get entirely taken over by the Nazis, standing idle? The West made it clear that they were never going to help anyone against the Nazis until it was their turn to be targeted.