The Molotov-Ribbentrop Pact and the Secret Protocol
On 23 August 1939, Germany and the Soviet Union signed a non-aggression pact that astonished the world. The public agreement committed both powers to neutrality if either became involved in war. But the document that mattered most was the secret protocol attached to it — a division of Eastern Europe into German and Soviet spheres of influence. Poland was to be partitioned along a line running roughly through the Narew, Vistula, and San rivers. Finland, Estonia, Latvia, and Bessarabia were allocated to the Soviet sphere. It was a diplomatic revolution achieved in weeks, and it made the German invasion of Poland on 1 September 1939 possible.
For Stalin, the pact appeared a strategic masterstroke. The Soviet Union would stay out of a war between Germany and the Western powers while securing territorial gains at no immediate military cost. The memory of catastrophic Russian losses in the First World War, and the fragility of the Red Army after the purges of its officer corps, made this calculation seem rational. What Stalin did not anticipate — or chose not to — was that Germany would defeat France in six weeks and that the buffer he was building would eventually become a killing ground.
The Soviet Invasion: 17 September 1939
Sixteen days after Germany invaded Poland from the west, the Red Army crossed Poland’s eastern border on 17 September 1939. The Soviet government announced it was acting to protect the Ukrainian and Belarusian minorities in eastern Poland — people who had, in Soviet framing, been oppressed under Polish rule. The announcement was carefully phrased to avoid a formal declaration of war and exploited genuine ethnic complexities in the region. But the reality was straightforward: the Soviet Union was claiming its share of the partition agreed in secret six weeks earlier.
The Polish army, already fighting desperately in the west, was wholly unable to defend two fronts simultaneously. Polish border guards and local garrisons mounted resistance in some areas, but organised defence against the Red Army was impossible. Within weeks, the Soviet Union had occupied eastern Poland — territories it designated as western Ukraine and western Belorussia, incorporating them into the Ukrainian and Belarusian Soviet Republics.
The Nature of Soviet Occupation
The Soviet occupation of eastern Poland was swift, systematic, and brutal. The NKVD followed the Red Army and immediately set about dismantling Polish state structures and eliminating those who represented them. Polish officers, policemen, civil servants, landowners, clergy, and professionals were identified, arrested, and processed. The Soviet system that had been applied to the USSR’s own population through the 1930s was now exported to a newly conquered territory.
Between 1939 and 1941, an estimated 320,000 to over a million Polish citizens were deported to Siberia, Kazakhstan, and other remote regions of the Soviet Union. The deportations were carried out in four main waves, the first in February 1940 — in winter, with families given hours to pack before being loaded onto freight trains. The conditions were lethal: inadequate food, no heat, and destinations where survival required brutal labour. Tens of thousands died in transit or in the first months of exile.
The Katyn Massacre
In spring 1940, the Politburo authorised the execution of approximately 22,000 Polish officers, police, civil servants, and members of the intelligentsia held as prisoners of war. Stalin signed the order on 5 March 1940. The killings took place at three main sites — Katyn Forest near Smolensk, Kalinin, and Kharkiv — over several weeks in April and May 1940. The victims were shot in the back of the head, individually, by NKVD executioners. The bodies were buried in mass graves.
When Germany invaded the Soviet Union in 1941 and occupied the relevant territory, German forces discovered the graves at Katyn in 1943 and publicised the discovery as evidence of Soviet atrocities. The Soviet government denied responsibility and blamed the Germans. This denial was maintained throughout the Cold War, sustained by political necessity and the complicity of Western governments who found it convenient to avoid the question. The truth was not officially acknowledged until 1990, when Mikhail Gorbachev admitted Soviet responsibility and released the original Politburo order bearing Stalin’s signature.
The Polish Citizens Deported East
The deportees who survived the journey found themselves in conditions designed to break them. Special settlements in Kazakhstan and Siberia imposed forced labour, restricted movement, and systematic deprivation. Families were separated, children sent to Soviet orphanages, and adults assigned to collective farms or labour sites where food rations were tied to work output. Many of those deported were women, children, and elderly relatives of the men already arrested or killed — the Soviet system targeted entire family networks rather than individuals alone.
The fate of these deportees changed dramatically after June 1941, when Germany invaded the Soviet Union and the Polish government-in-exile in London signed an agreement with Stalin. Polish citizens were technically freed from Soviet custody and allowed to form a Polish army under General Wladyslaw Anders. The Anders Army, assembled from the survivors of the deportations and camps, eventually fought its way through Iran, North Africa, and Italy. The soldiers of Monte Cassino included men who had survived Soviet labour camps.
The Legacy of 1939
The Soviet invasion and occupation of eastern Poland left wounds in Polish national memory that decades of communist rule suppressed but could not heal. The Katyn massacre in particular became a defining trauma — not only because of the scale of the killings but because of the decades of denial. Polish families who asked what had happened to fathers, brothers, and husbands were lied to by a state that claimed to be their ally. The lies were enforced through the communist government installed in Poland after 1945, which could not acknowledge Soviet atrocities without undermining its own legitimacy.
The events of 1939 also shaped the post-war settlement in ways still felt today. The eastern territories absorbed by the Soviet Union in 1939 — including the city of Lwow, now Lviv in Ukraine — were not returned to Poland after 1945. Instead, Poland’s borders were shifted westward: losing territory in the east, gaining it from Germany in the west. Millions of people were displaced in the process. The Soviet invasion of September 1939 was the first step in a chain of territorial and demographic transformations that remade the map of Eastern Europe permanently.