On September 17, 1939, sixteen days after Nazi Germany invaded Poland from the west and without a formal declaration of war, the Soviet Union invaded Poland from the east under the terms dictated by the Molotov–Ribbentrop Pact.
After more than a century of partition between the Russian Empire, the Kingdom of Prussia and the Habsburg Monarchy, Poland re-emerged as a sovereign country at the end of the First World War. But until the Paris Peace Conference and Treaty of Versailles formally established Poland as an independant state, many territorial issues remained unresolved.
Poland, Germany and Russia
Under the leadership of its new Chief of State Józef Piłsudski, the young Polish state solidified its independence in a series of conflicts that re-defined the country’s borders as it sought to expand as far east as possible in an attempt to create a Polish-led federation, capable of countering future imperialist action on the part of Russia or Germany.
But by 1920, the Bolsheviks had gained the upper hand in the Russian Civil War and Vladimir Lenin began viewing Poland as a bridge to bring communism to Central and Western Europe. As the Red Army advanced westward, border skirmishes eventually culminated in all-out war, but the Polish victory at the Battle of Warsaw sealed the fate of the Soviets, who sued for peace.
After the resounding success of the “Miracle on the Vistula”, the young Polish state even gained 200km of territory east of its former borders when a peace treaty was signed in Riga in 1921, dividing the disputed territories between Poland and Soviet Russia and securing the Polish state’s eastern frontiers until 1939.
But war began brewing once more during the 1930s with the Nazi Party’s rise to power in Germany. Territorial losses following the Treaty of Versailles had indeed incited German revanchism, particularly regarding the status of the Free City of Danzig and of the Polish Corridor, and attending to these issues became part of Adolf Hitler’s political platform.
Following the Anschluss of Austria in March 1938 and the conquest of Czechoslovakia in March 1939, the Nazi leader turned his sight to Poland.
The Molotov–Ribbentrop Pact
Hitler’s rise to power also increased tensions between Germany and the Soviet Union. With Europe on the brink of another major war, Soviet leader Joseph Stalin started to view an alliance with Hitler as a plausible way to keep his nation on peaceful terms with the Nazi state. And by early August 1939, Germany and the Soviet Union started to discuss a political alliance.
Signed in Moscow in August 1939 by the German and Soviet foreign ministers Joachim von Ribbentrop and Vyacheslav Molotov, the Treaty of Non-Aggression between Germany and the Soviet Union known as the the Molotov–Ribbentrop Pact provided a written guarantee of peace by each party towards the other.
But in addition to the publicly-announced stipulations, the treaty also included a Secret Protocol, which provided for the partition of Poland and the rest of Central and Eastern Europe into Soviet and German “spheres of influence”.
A week later, Hitler ordered his troops to strike east into Poland. World War II had begun.
Soviet invasion and occupation of Poland
Sixteen days after Nazi Germany invaded Poland from the west and without a formal declaration of war, the Red Army entered the eastern regions of Poland with seven field armies, containing between 450,000 and 1,000,000 troops, split between two fronts.
The Red Army, which vastly outnumbered the Polish defenders, achieved its targets encountering only limited resistance, capturing some 320,000 Polish prisoners of war in the process.
A campaign of mass persecution in the newly acquired areas began immediately and in November 1939, the Soviet government annexed the entire Polish territory under its control. Some 13.5 million Polish citizens who fell under the military occupation were made into new Soviet subjects following show elections conducted by the NKVD secret police in the atmosphere of terror.
Embodied by the Katyń Massacre, political murders and other forms of repression, targeting Polish figures of authority such as military officers, police and priests, began with a wave of arrests and summary executions. Meanwhile, the NKVD sent hundreds of thousands of people from eastern Poland to Siberia and other remote parts of the Soviet Union in four major waves of deportation.
The Eastern Front
Soviet forces occupied eastern Poland until the summer of 1941, when they were driven out by the German army in the course of Operation Barbarossa. By then, the entirety of Poland had fallen under German rule, and the Nazis proceeded to advance their genocidal policies across the country.
The area remained under German occupation until the Red Army reconquered it in the summer of 1944. With the Soviet forces closing in on the Polish capital, the Western-backed Polish Home Army, hoping to liberate the city before the Soviet-backed Committee of National Liberation could assume control, launched the Warsaw Uprising to free the Polish capital from German occupation.
But the advancing Soviet forces temporarily halted combat operations, enabling the Germans to regroup and send in reinforcements. This led to allegations that Stalin tactically halted his forces to let the Warsaw Uprising fail and allow the pro-Soviet Polish administration, rather than the Polish government-in-exile, to gain control of Poland.
Outnumbering the German forces five to one, the Red Army eventually launched the massive Vistula–Oder Offensive across the Vistula river in January 1945. And in April, the Red Army jumped off from lines on the Oder and Neisse rivers, marking the start of the Battle of Berlin, which proved to be the culminating offensive of the war on the Eastern Front.
Legacy of the Molotov–Ribbentrop Pact
The Yalta Conference permitted the Soviet Union to annex almost all of their Molotov–Ribbentrop Pact portion of Poland, compensating the new Polish People’s Republic with the greater southern part of East Prussia and territories east of the Oder–Neisse line.
The Soviet Union appended the annexed territories to the Ukrainian Soviet Socialist Republic, the Byelorussian Soviet Socialist Republic and the Lithuanian Soviet Socialist Republic.
The rumour of the existence of the Secret Protocol was proved only when the German copy was found in the German archives and was made public during the Nuremberg Trials. But the Soviet government denied its existence until 1989, when Mikhail Gorbachev admitted and condemned its existence.
While Vladimir Putin has condemned the Molotov-Ribbentrop Pact as “immoral”, he has also defended it as a “necessary evil” even claiming that the signing of the Pact was no worse than the 1938 Munich Agreement, which led to the partition of Czechoslovakia.
Find out more about Central European history in our new On this Day series.