The official justification of the time and one still banded about by ideologues like Grover Furr or r/Communism was that they were coming to the rescue of eastern Poland's Ukrainian and Belorussian minorities. This form of proactive protection was necessary according to the Soviets because the war created a vacuum in which these groups were in danger. Additionally, the Soviets also contended that its invasion was not an invasion, because the Polish state and its government had ceased to exist because of the German invasion. This rather curious position meant that the Soviets were no longer bound by the Soviet-Polish non-aggression treaty and this formerly Polish space was now neutral territory.

This is of course, self-serving nonsense. There is considerable evidence that exists that the Soviets explicitly timed their invasion after the German invasion to give this justification. The Soviet Foreign Minister Molotov colluded with the German ambassador to the USSR, Friedrich Werner von der Schulenburg, over the timing and the German minister protested that the Soviets explicitly cited the German threat as a pretext. Von der Schulenberg managed to get the direct references to Germany deleted from the official pronouncement and the Soviets instead emphasized that they were acting out of fraternal concern for their Ukrainian and Belorussian compatriots from some vague threat.

After the Second World War, the Soviet justification for the Pact and subsequent invasion shifted slightly. The official line still stressed the fundamentally altruistic motives for Soviet actions and held that this region was not really "Polish", but rather Belorussian or Ukrainian and only ended up a part of the Polish by counter-revolutionary accident. In light of Barbarossa and the Cold War, the Soviets argued the Pact was necessary to buy the USSR time, especially since the Western democracies had displayed their reluctance to ally with the Soviet Union during the Sudeten Crisis of 1938. This was also a politicized and selective reading of events that relies upon taking kernels of facts and magnifying them all out of proportion. The Soviet narrative also selectively forgot that between 1939 and Barbarossa, its official position on the war was this was an imperialist war and that Britain and France were the aggressors, not Germany. This created massive confusion among the global left and caused many leftists to finally break their ideological affinity with the Soviet Union.