For Poland, the part they invaded had only been part of Poland for about 20 years and it was part of the Ukrainian and Belarusian SSRs until the Polish-Soviet war in the early 20s. Poland was also not as innocent as most people say, as it had a non-aggression pact with Germany for years before the Molotov-Ribbentrop pact and participated in the annexation of Czechoslovakia.

As for the Winter War, the situation was such that it had become clear that Finland, which was ruled by a brutal anti-communist bourgeois government that regularly imprisoned and murdered socialists as well as encouraged frequent raids by Whites and other armed terrorist groups into Soviet territory, cross-border attacks on villages in Russian Karelia, etc. was closely allied with Nazi Germany and would pose a severe threat to the security of the Soviet Union in case of a conflict since they would at the very least allow German troops to use Finnish territory as a staging ground for invasion.

Stalin made numerous attempts to resolve the security issue around Leningrad - the second most important Russian city - diplomatically, offering greater land concessions in Karelia in exchange for an adjustment of the Finnish-Soviet border a few kilometers away from the city as well as guarantees that the islands along the coast in the Gulf of Finland would not be militarized. The British got wind of this and secretly encouraged the Finns to completely stonewall the Soviets and refuse any deal. In return they promised that they would help Finland in case of an armed conflict (sound familiar?).

It is unclear which side started the war as there were reports of Finnish attacks on Soviet border posts, but either way the situation had become an unacceptable security risk (Stalin’s worries were vindicated as the Finns and the Nazis would indeed only a couple of years later invade and jointly siege Leningrad killing over a million civilians, many of these dying of starvation). The war went about as can be expected, the British chickened out and did not keep their promise, though they did send weapons and ammunition to Finland, the Soviets struggled a bit initially until they broke through the Mannerheim line of fortifications and the Finns sued for peace when it became clear they were about to be encircled and overrun.

Stalin took almost exactly what he had asked for before the war, plus a tiny bit more for security (a small strip of Karelia, basically empty forests in the tundra, and some small islands along the Gulf of Finland to prevent an enemy navy sailing right up to Leningrad). All throughout this conflict the Finnish media was banned from visiting the front and government propaganda maintained they were winning handily right up until they were defeated. The Western bourgeois governments made a big fuss over the conflict, the League of Nations broke its own rules and expelled the Soviets despite only 7 out of 15 nations on the Council voting to do so (and those seven included Britain, France and Belgium, as well as three countries that had just been added the day before) and western newspapers spread a ton of fake news about how poorly the Soviets allegedly performed and how heroic the Finns were (again, remind you of anything today?).

There was no “Soviet invasion of Poland”, even just that framing is a pervasive and pernicious bit of anti-Soviet propaganda and falsification of history that has unfortunately become too uncritically accepted.

The Soviet Union never invaded Poland because Poland had formally already ceased to exist as a state after its government fled into exile in Romania - a neutral country at the time. There was never a declaration of war by the USSR on Poland or vice versa in 1939, nor was there any legal order given by the Polish leadership to resist the Soviet troops, and furthermore all territories that the Soviets occupied to prevent the Nazis from marching into them unopposed after the Polish army collapsed were territories belonging to the Belarussian and Ukrainian Soviet Republics that had been invaded and illegally annexed by Poland in 1920.

The peace treaties that were signed following the conclusion of WW1 had established Poland as an independent state within certain borders that were drawn such as to include only Polish majority lands. However the Polish government during the inter war period was highly reactionary, ultra-nationalist, militaristic and proto-fascist. It was obsessed with expanding into the “Intermarium” to recreate the borders of the former Polish-Lithuanian Commonwealth.

They soon took advantage of the fact that Russia was embroiled in a civil war to invade, occupy and subsequently spend the next two decades brutally repressing and attempting to replace the majority Byelorussian and Ukrainian population in this new so-called “Eastern Poland” via what was essentially a colonial land theft in which they would install Polish landlords over the native population. This led among other things to the radicalization of Ukrainian nationalists who would go on to join the Nazis and commit some of the worst massacres and atrocities of the entire second World War against Poles, Jews, and even other Ukrainians.

Lenin and the Red Army resisted the initial Polish invasion and managed to retake a large chunk of Ukraine back from the Poles, including Kiev, until the unfortunate turn of events at the Vistula stopped the Red Army. Stalin merely took back the rest of the stolen land Poland had managed to keep.

Fun fact, at the same time as their attack on the Ukraine and Belarus regions in the aftermath of WW1, Poland also invaded Lithuania in this expansionist crusade that they decided to go on after they were granted independence. This is all conveniently forgotten nowadays.

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