I've been listening to Proles Pod, they have a new series of episodes called "The Stalin Eras" which I found extremely good for history of the Soviet Union from 1917 to the end of the Great Patriotic War. Using that as a source and a few other sources, I've compiled some main points regarding the Motherboard-Ribbedcock that dispels the prevalent propaganda that it was a "Soviet-Nazi pact to expand the Soviet Union because they were bad". I've used mostly Wikipedia in the links so you can use it against libs:
1) Most of the invaded "Polish" territories actually belong to modern Lithuania, Ukraine and Belarus. In 1919, Poland started the Polish-Ukrainian war and invaded Ukraine, Belarus and part of the RSFSR. This so-called "carving of Poland by the Soviet Union" liberated many formerly oppressed non-Polish national ethnicities such as Lithuanians in Polish-controlled Vilnius arguably being genocided, or ceding the city of Lviv to the Ukraine SSR. Sorry for the ugly map, I made it myself and it's my first attempt (I made it with GIMP lmao):
Edit: added the following map (source) showing the majority-ethnicities in 1931-Poland for further reference. Funny how, comparing both maps, the rough boundary between Polish and Ukrainian/Russian/Belarusian ethnic majority seems to really overlap with the extent to which the USSR invaded Poland 
2) The Soviet Union had been trying for the entire 1930s to establish a mutual-defense agreement with Poland, France and Britain against the Nazis, under the doctrine of the then-People's Commisar of Foreign Affairs Maxim Litvinov. This decade-long proposal for mutual-defence went completely ignored by France and England, which hoped to see a Nazi-Soviet conflict that would destroy both countries, and Poland didn't agree to negotiations by itself either. The Soviet government went as far as to offer to send one million troops together with artillery, tanking and aviation, to Poland and France. The response was ignoring these pleas and offerings.
Furthermore, this armistice between the Soviet Union and Nazi Germany happened only one year after the Munich Betrayal. The Soviet Union and France had a Mutual Defense Agreement with Czechoslovakia, which France (together with the UK) unilaterally violated in agreement with the Nazis when ceding Czechoslovak territories to Nazi Germany. Stalin offered France, as an alternative to the Munich Betrayals, a coordinated and two-front attack to Nazi Germany, which France rejected in favour of the Munich Agreements.
3) The Soviet Union had been through WW1 up to 1917, the Russian Civil War up to 1922 (including a famine that killed millions) in which western powers like France, England or the USA invaded the Bolsheviks and helped the tsarist Whites to reestablish tsarism, which ultimately ended with a costly Bolshevik victory; the many deaths of famine during the land-collectivization of 1929-1933, and up to 1929 was a mostly feudal empire with little to no industry to speak of. Only after the 1929 and 1934 5-year plans did the USSR manage to slightly industrialize, but these 10 years of industrialization were barely anything in comparison with the 100 years of industrialization Nazi Germany enjoyed. The Soviet Union in 1939 was utterly underdeveloped to face Nazi Germany alone, as proven further by the 27 million casualties in the war that ended Nazism. The fact that the Soviet Union "carved Eastern Europe" in the so-called "secret protocol" was mostly in self-defense. The geography of the Great European Plain made it extremely difficult to have any meaningful defenses against Nazis with weaponry and technological superiority, again proven by the fact that the first meaningful victory against Nazis was not in open field but in the battle of Stalingrad, which consisted more of a siege of a city. The Soviet Union, out of self-preservation, wanted to simply add more Soviet-controlled distance between themselves and the Nazis. You don't have to take my word for all of this, you can hear it from western diplomats and officials from the period itself. I hope nonbody will find my choice of personalities to reflect a pro-Soviet bias (I have another post with many more quotes, these are just a few of them):
“In those days the Soviet Government had grave reason to fear that they would be left one-on-one to face the Nazi fury. Stalin took measures which no free democracy could regard otherwise than with distaste. Yet I never doubted myself that his cardinal aim had been to hold the German armies off from Russia for as long as might be” (Paraphrased from Churchill’s December 1944 remarks in the House of Commons.)
“It would be unwise to assume Stalin approves of Hitler’s aggression. Probably the Soviet Government has merely sought a delaying tactic, not wanting to be the next victim. They will have a rude awakening, but they think, at least for now, they can keep the wolf from the door” Franklin D. Roosevelt (President of the United States, 1933–1945), from Harold L. Ickes’s diary entries, early September 1939. Ickes’s diaries are published as The Secret Diary of Harold Ickes.
"One must suppose that the Soviet Government, seeing no immediate prospect of real support from outside, decided to make its own arrangements for self‑defence, however unpalatable such an agreement might appear. We in this House cannot be astonished that a government acting solely on grounds of power politics should take that course” Neville Chamberlain House of Commons Statement, August 24, 1939 (one day after pact's signing)
"It seemed to me that the Soviet leaders believed conflict with Nazi Germany was inescapable. But, lacking clear assurances of military partnership from England and France, they resolved that a ‘breathing spell’ was urgently needed. In that sense, the pact with Germany was a temporary expedient to keep the wolf from the door” Joseph E. Davies (U.S. Ambassador to the USSR, 1937–1938) Mission to Moscow (1941)
I could go on with quotes but you get my point.
4) The Soviet Union invaded Poland 2 weeks after the Nazis, at a time when there was no functioning Polish government anymore. Given the total crushing of the Polish forces by the Nazis and the rejection of a mutual-defense agreement from England and France with the Soviets, there is only one alternative to Soviet occupation of Eastern Poland: Nazi occupation of Eastern Poland. Seriously, what was the alternative, letting Nazis genocide even further east, killing arguably millions more in the process over these two years between Molotov-Ribbentrop and Operation Barbarossa? France and England, which did have a mutual-defense agreement with Poland, initiated war against Germany as a consequence of the Nazi invasion, but famously did not start war against the Soviets, the main reason in my opinion being the completely different character of the Soviet invasion. Regardless of this, please tell me. After the rejection of mutual-defense agreements with the Soviet Union: what was the alternative other than Nazi occupation of Eastern Poland?
Edit 2: 5) I, the guy who wrote this wall of text, am a Spaniard. The Soviet Union is the only country which sold weapons to and supported the antifascist side of the Spanish civil war in 1936-1939. The Soviet Union not only declared opposition to fascism in Europe, it is the only country pre-1939 to actually fight it outside its borders. While the Italian Fascists and the German Nazis bombed the cities of the republican-controlled areas of Spain, the liberal west looked to the other side, and the USSR was the only country to offer material support and actual troops to the Spanish partisans. So, as a Spaniard, fuck you if you diminish the role of the USSR in the antifascist struggle in Europe.
Thanks for reading :)
