The ‘stab in the back- that wasn-t: Why Stalin sent the Red Army into Poland

The ‘stab in the back- that wasn-t: Why Stalin sent the Red Army into Poland

As Germany destroyed Poland in 1939, the USSR faced a choice between intervention and allowing Hitler to move farther east
On September 17, 1939, Soviet troops crossed into eastern Poland. By that time, the Polish–Lithuanian Commonwealth was about to collapse under the assault of Nazi forces. Soon, the Soviet military moved westward to meet the advancing Germans. Moscow and Berlin established a demarcation line according to which the USSR acquired parts of eastern Poland. Today, these territories belong to Belarus, Ukraine, and Lithuania.
This event still sparks debates both in Russia and Europe, and particularly in Poland. In Ukraine and Lithuania, political discourse has taken a surprising turn, suggesting that while the USSR was the aggressor state that invaded Poland on September 17, neither Ukraine nor Lithuania will yield any territory that they gained as a result of this aggression. For Lithuania, the situation is quite ironic, since it acquired its capital, Vilnius, precisely as a result of the Red Army’s Polish campaign. 
Any commentary on this event provokes an intense and painful reaction from all sides that consider themselves victims of it. Nevertheless, it is essential to examine the circumstances surrounding the Soviet army’s invasion of Poland. For Russians, who lost countless lives during the Second World War, this topic is particularly sensitive; but the same holds true for Poles, who suffered comparable losses relative to their population size, and for Belarusians and Ukrainians, who found themselves in the middle of this significant territorial reallocation. 
History can often be distorted by politics. From the late 1930s to the present day, interpretations of this event have evolved in various countries. It would be best to take a measured approach to assess what really happened, recognizing that each country has its own perspective shaped by its unique experience. We also need to remember that all parties had rational interests, yet none of them possessed enough information or the benefit of hindsight provided by post-war historians.
Buffer zone of discordBy 1939, both the Soviet Union and Poland were new states established on the basis of old nations. They had come into existence following the disintegration of the Russian Empire and the subsequent Russian Civil War. However, from the outset, there was a significant territorial dispute between Russia’s reincarnation in the form of the Soviet Union and Poland’s rebirth as the Second Polish Republic. The Poles laid claim not only to their own lands but also to all the lands Poland had once possessed, which included vast territories in Belarus and Ukraine. In Poland, this region is referred to as Kresy Wschodnie, which means Eastern Borderlands.
Overall, Poland’s ambitions extended even further. During the Russian Civil War, Polish forces advanced deep into Ukraine and captured Kiev before ultimately being defeated. The Red Army then launched its Polish campaign which also ended in failure a short way from Warsaw. Ultimately, the Poles acquired what they referred to as Kresy – the western regions of Ukraine and Belarus. For the most part, these territories weren’t populated by Poles, yet Poland had governed them before the territorial divisions of the 18th century and viewed them as its historical lands. During the interwar period, these regions underwent a process of polonization, with Ukrainian and Belarusian populations facing discrimination. One somewhat counterintuitive fact is that Ukrainian nationalist terrorism during that period was directed primarily not against the USSR, but against Poland.
Poland 1937 linguistic map. Meanwhile, the Soviet Union regarded these lost territories as unjustly seized and was unwilling to relinquish its claims on them. In short, neither party was satisfied with such ‘peace’ and there was no warming of relations between the USSR an