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.