Soviet Invasion of Poland (1939): The Nazi-Soviet Partition

The Soviet invasion of Poland began on 17 September 1939, when the Red Army crossed Poland’s eastern border while the Polish army was fighting the German invasion in the west. Carried out under a secret protocol of the Molotov-Ribbentrop Pact, it completed the partition of Poland between Nazi Germany and the Soviet Union. The occupation that followed brought mass arrests, deportations and the Katyn massacre.

Background: the secret protocol

On 23 August 1939 Germany and the Soviet Union signed a non-aggression pact in Moscow, negotiated by foreign ministers Joachim von Ribbentrop and Vyacheslav Molotov. A secret additional protocol divided Eastern Europe into spheres of influence, with Poland split roughly along the Narew, Vistula and San rivers. The Soviet Union denied the protocol’s existence for fifty years, acknowledging it only in 1989.

When Germany began its invasion of Poland on 1 September, Berlin urged Moscow to move quickly. Stalin waited until the Polish army was largely broken and until he had secured a ceasefire with Japan after the fighting at Khalkhin Gol, which came into effect on 16 September.

The invasion

Early on 17 September Molotov handed the Polish ambassador in Moscow a note claiming that the Polish state had ceased to exist and that the Soviet Union must protect the Ukrainian and Belarusian populations of eastern Poland. Hours later two Soviet fronts, the Belorussian Front under Mikhail Kovalev and the Ukrainian Front under Semyon Timoshenko, crossed the border with several hundred thousand troops, with estimates commonly ranging from about 450,000 to over 600,000.

The eastern frontier was held mainly by the thinly spread Border Protection Corps. Marshal Edward Rydz-Śmigły ordered Polish forces not to fight the Soviets except in self-defence or to resist disarmament, but many units fought anyway. There was heavy fighting in the defence of Grodno from 20 to 22 September, as well as at Wilno and at Szack on 28 September. Lwów, besieged by German troops, surrendered to the Red Army on 22 September. In several places Soviet troops and local militias murdered prisoners, officials and landowners.

On 22 September German and Soviet forces held a joint ceremony at Brest-Litovsk as the city was handed to the Red Army, overseen by General Heinz Guderian and Soviet brigade commander Semyon Krivoshein. On 28 September the two powers signed a Boundary and Friendship Treaty that moved the dividing line east to the Bug river, in return for placing Lithuania in the Soviet sphere.

Annexation and occupation

The Soviet Union took about half of Poland’s territory, with some 13 million inhabitants, including Poles, Ukrainians, Belarusians and Jews. After staged elections to “people’s assemblies” on 22 October, the regions were formally incorporated into the Ukrainian and Belorussian Soviet republics in November 1939. The city of Vilnius and its surroundings were handed to Lithuania, itself annexed by the Soviet Union in 1940.

Soviet rule brought the confiscation of property, the closure of Polish institutions and widespread arrests by the NKVD. Between 1940 and 1941 several waves of deportations sent hundreds of thousands of people to labour camps and settlements in Siberia and Kazakhstan; estimates range from about 320,000 in Soviet records to around one million in some Polish estimates. In spring 1940 the NKVD murdered about 22,000 Polish officers, police and members of the elite in the Katyn massacre and at other sites.

Later history and memory

The German-Soviet partition lasted until June 1941, when Germany attacked the Soviet Union and overran the former Polish east within weeks. After the war, at the conferences of Tehran and Yalta, the Allies accepted a new Polish-Soviet frontier roughly following the Curzon Line, a boundary first proposed in 1919. Poland was compensated with former German territory in the west, and millions of Poles, Ukrainians and Germans were resettled.

In communist Poland the events of 17 September were a forbidden subject, and official histories avoided describing them as an invasion. Only after 1989 could the date be openly commemorated. The Soviet government acknowledged responsibility for Katyn in 1990, but interpretations of the 1939 invasion remain contested between Poland and Russia.

By the numbers

Timeline

• 1939, 23 August: The Molotov-Ribbentrop Pact is signed with its secret protocol on Poland.
• 1939, 17 September: The Red Army invades eastern Poland.
• 1939, 22 September: Lwów surrenders; German and Soviet troops meet at Brest-Litovsk.
• 1939, 28 September: The German-Soviet Boundary and Friendship Treaty is signed.
• 1939, 22 October: Staged elections are held in the occupied territories.
• 1939, November: The territories are annexed into the Ukrainian and Belorussian Soviet republics.
• 1940, April–May: The Katyn massacre takes place.

Why it matters

The Soviet invasion made the destruction of Poland in 1939 a joint Nazi-Soviet act, a fact long suppressed in the Soviet bloc. It pushed the Soviet border west, set the stage for the Winter War and the occupation of the Baltic states, and left a legacy of deportation and killing that still shapes relations between Poland and its eastern neighbours.