It is often said that Stalin or the Soviet government entered into a treaty with Germany in order to divide Poland and seize the Baltic states. Thе treaty was signed on the basis of national interests. But USSR had neither documents nor facts showing that Hitler would definitely attack Poland. There were intelligence reports – this was assumed, but there was no certainty. USSR certainly was not going to divide Poland.
A recurring disinformation narrative revising the Molotov-Ribbentrop Pact.
Molotov-Ribbentrop Pact (the Treaty of Non-Aggression between Germany and the Union of Soviet Socialist Republics) was signed on August 23, 1939. To the public pact of nonaggression was appended a secret protocol, which divided the whole of eastern Europe into German and Soviet spheres of influence. Poland east of the line formed by the Narew, Vistula, and San rivers would fall under the Soviet sphere of influence. The protocol also assigned Lithuania, Latvia, Estonia, and Finland to the Soviet sphere of influence.
In 1989, the Soviet Congress of People's Deputies condemned these protocols: "The content of this treaty did not diverge with the norms of international law. However, during the signing of the treaty and in the process of its ratification the signature of an "additional secret protocol", which delimited the "areas of interest" of the parties from the Baltic to the Black Sea, from Finland to Bessarabia, was hidden."
The original documents were burned by the Nazis at the end of the war, but a microfilm copy was saved and turned over to British and American authorities, who revealed its contents. Read more about this in the New York Times article.