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.

The rationale behind this pre-Barbarossa narrative is very nebulous and Stalin tended to play his diplomatic cards close to his chest. One interpretation is that Stalin wanted to gain as much assistance from Germany as possible in light of a realistic assessment of Hitler's long-term intentions. The Molotov-Ribbentrop Pact included provisions for German technical assistance and exchange while the USSR provided the German war economy much needed natural resources. Another interpretation of Stalin's actions was that he viewed the outbreak of war in Marxist-Leninist terms. War between the Western allies and Germany was the harbinger of a final internal struggle of imperialist capitalism. The Soviet's backing of the weaker power ensured that this struggle would be protracted and pave the way for a Soviet-style revolution throughout Europe. This was the prediction Stalin gave to the Comintern chief Georgi Dimitrov, who recorded in his diary:

A war between two groups of capitalist countries (poor and rich as regards colonies, raw materials and so forth)- for the redistribution of the world, for the domination of the world! We see nothing wrong with their having a good fight and weakening each other. It would be as fine at the hands of Germany the richest capitalist countries (especially England) were shaken. Hitler, without understanding it or desiring it, is shaking and undermining the capitalist system.

Dimitrov would explain in various communiques to Communist Parties that while the 1930s Popular Front strategy of allying with Western democratic parties to combat fascism was correct, within the context of war, the ideologically correct view was to see both sides as imperialist aggressors. Communist activism was to highlight the bourgeois profiteering of the Western powers, both of whom controlled large global empires. Within this context, painting Germany as a relatively innocent victim underscored the true nature of the division of the global economy.

This type of double-think played havoc among the various European communist parties. The French Communist Party had anticipated a resurgence of the Popular Front with the outbreak of war on 1 September and the Soviet invasion of 17 September and directives caught its leadership flat-footed. The Comintern forced the resignation of the general secretary of Great Britain's Communist Party, Harry Pollitt when the latter refused to tow the party line that Germany was an aggressor. Spanish and German Communist emigres in the USSR had to swallow this narrative or risk offending their Soviet hosts. Although a good many Communist Party leaders followed Moscow's tune, there was a great deal of dissatisfaction within the rank and file and in a few cases, open breaks with Moscow. But for the most part, the leadership of the Communist Parties tended to obey orders and sought to actively hinder the Anglo-French war effort. These efforts met with only minor success as the support for Germany alienated many Communists from their left-wing allies as well as their own rank and file. This fractured relationship in the left would not be repaired until Barbarossa, but memory of their would remain an important reason for a number of mainstream socialist parties, especially in Germany, rejection of political alliances with Communist Parties postwar.