Mao, on the other hand only took up arms against Japan.

An invading and occupying imperialist force that was busily conducting war crimes. How the hell did you expect China to aid the Western front, exactly? The West was content to let Japan do whatever it wanted to the rest of East Asia so long as they didn't attack their colonial holdings. Only then did they actually step in.

Trotting out with the damn Molotrov-Ribbentrop Pact is old hat by this point, but now you're inventing new ways to be anticommunist. I'd almost be impressed if I weren't so thoroughly disgusted.

The communists were never allies with the Nazis. A non-aggression pact is not an alliance. 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.

Churchill did not take the Nazis as a serious threat, and was horrified when FDR and Stalin made a joke about executing Nazis. Churchill starved millions to death in India in preventable ways, and had this to say about it:

I hate Indians. They are a beastly people with a beastly religion. The famine was their own fault for breeding like rabbits.

Meanwhile, the soviet famine in the 1930s was the last major famine outside of wartime in the USSR, because collectivized farming achieved food security in a region where famine was common. As a consequence, life expectancy doubled:

The Nazis and soviets were never allies. A non-aggression pact is not an alliance, and the non-aggression pact between the soviets and the Nazis was unique among the other non-aggression pacts in that it was on the eve of war. The soviets knew war was coming, and so bought more time to prepare.

Your knowledge of history matches your knowledge of Stalin and Mao (inadequate is a fitting term).

The non aggression pact was necessary to delay the inevitable long enough to industrialise post civil war and build up a force to fight the Nazis largely alone as the western powers had continuously refused to form an anti nazi pact since 1933. The soviets were also the last major power to sign a non aggression pact with the Nazis. The USSR broke the nazi beast took the majority of the casualties and killed the majority of the Nazis.

The USSR spent years trying to build an anti-Nazi alliance. 1933 they proposed collective security at the League of Nations. 1935 they signed mutual defense pacts with France and Czechoslovakia. Spring 1939 they sat in Moscow for months begging Britain and France for a real triple alliance. The West stalled, refused to guarantee the Baltics, refused to let the Red Army cross Poland to actually fight Hitler. Poland’s elite, more scared of workers than of Nazis, said no too and instead joined Hitler in attacking Czechoslovakia. The Molotov-Ribbentrop Pact happened because liberals handed Hitler Eastern Europe rather than work with socialists.

When Soviet troops entered eastern Poland September 17 1939, the Polish state had already collapsed. Government fled to Romania September 15. Warsaw was burning. The army was broken. The lands the USSR moved into? Not Poland proper. Territories Poland had seized and occupied by force in 1919-1921 from Belarus, Ukraine, and Lithuania. Moving in to secure this land also saved millions from extermination.