The invasion of Poland, which began on September 1, 1939, was the joint attack on the Second Polish Republic by Nazi Germany and, from September 17, the Soviet Union, and it is universally recognized as the event that triggered World War II in Europe. The campaign, which Poland’s allies Britain and France responded to with declarations of war on Germany, ended in early October 1939 with the complete partition of Polish territory between the two invaders.
Background: Why Poland Was Targeted
Poland’s geographic position made it a strategic prize and a buffer state between Germany and the Soviet Union. After World War I, the Treaty of Versailles had created an independent Poland from territories that had belonged to the German, Austro-Hungarian, and Russian empires. This settlement left Germany with the so-called Polish Corridor, a strip of land giving Poland access to the Baltic Sea, which separated East Prussia from the rest of Germany. The port city of Danzig (now Gdańsk) was declared a free city under League of Nations protection, but its population was predominantly German and its status was a constant source of friction.
Adolf Hitler, who came to power in 1933, made the revision of the Treaty of Versailles a central goal. He demanded the return of Danzig and the construction of an extraterritorial highway and railway across the Corridor. The Polish government, wary of German intentions and bound by a mutual defense treaty with France and a guarantee from Britain, refused these demands. Hitler, having already annexed Austria and the Sudetenland in 1938, decided that a military solution was preferable to further negotiation. In the secret Molotov-Ribbentrop Pact, signed on August 23, 1939, Nazi Germany and the Soviet Union agreed to divide Poland between them and to not attack each other, clearing the way for the invasion.
The German Plan: Blitzkrieg
The German strategy for the invasion was based on the concept of Blitzkrieg (lightning war), which emphasized speed, surprise, and the concentration of armor and air power. The plan, known as Fall Weiss (Case White), called for a series of concentric attacks from the north, west, and south, aimed at encircling and destroying the Polish army west of the Vistula River.
- Army Group North attacked from Pomerania and East Prussia, aiming to cut the Polish Corridor and advance toward Warsaw.
- Army Group South attacked from Silesia and Slovakia, with the main armored thrust directed at Warsaw and Łódź.
- The Luftwaffe was tasked with achieving air superiority, disrupting Polish mobilization, and bombing strategic targets, including cities.
The German forces comprised about 1.5 million men, 2,700 tanks, and over 2,000 aircraft. They were organized into five armies under the overall command of General Walther von Brauchitsch, with Hitler as supreme commander.
The Polish Defense
Poland could field a similar number of soldiers—about 1 million—but was at a severe disadvantage in equipment and mobility. The Polish army had about 870 tanks (mostly light tanks and tankettes), 430 aircraft (many obsolete), and a large cavalry force. The Polish plan, Plan Zachód (Plan West), assumed that the main German attack would come from the west and that France and Britain would launch an offensive in the west, which they did not do in any meaningful way. The Polish forces were deployed in a long, thin line along the borders, which made them vulnerable to encirclement.
The Polish commander-in-chief, Marshal Edward Rydz-Śmigły, hoped to fight a delaying action while falling back to the Vistula and San rivers, but the speed of the German advance made this impossible. The Polish air force was largely destroyed on the ground in the first days, though some planes managed to escape to Romania or were used in a limited defensive role.
The Course of the Campaign
The invasion began at dawn on September 1, 1939, with the German battleship Schleswig-Holstein firing on the Polish garrison at Westerplatte, near Danzig. Simultaneously, German forces crossed the border at several points. The Polish army fought bravely but was unable to stop the coordinated attacks. The Battle of the Bzura River (September 9–19) was the largest Polish counterattack, but it ended in defeat. By September 17, the Germans had reached Warsaw and were besieging the capital.
On September 17, the Soviet Union invaded Poland from the east, citing the collapse of the Polish state and the need to protect ethnic Ukrainians and Belarusians. The Polish government and military leadership crossed into Romania on September 17–18, where they were interned. Warsaw surrendered on September 27, and the last major Polish resistance ended on October 6 with the Battle of Kock. The campaign lasted just 35 days.
Britain and France Respond
Britain and France had issued ultimatums to Germany, demanding withdrawal. When no response came, Britain declared war on Germany on September 3, 1939, followed by France later that day. Australia, New Zealand, India, and South Africa also declared war within days. However, the Allies did not launch a major offensive against Germany’s western border. The French army staged a brief advance into the Saarland in September but withdrew, and the period became known as the Phoney War. This inaction allowed Germany to concentrate its forces in Poland.
Aftermath and Occupation
Poland was divided between Germany and the Soviet Union along the line agreed in the Molotov-Ribbentrop Pact (the Curzon Line, roughly along the Bug River). The western part was incorporated directly into the German Reich, while the central part became the General Government, a German-administered zone. The Soviet-occupied east was annexed to the Soviet republics of Ukraine and Belarus.
The occupation was brutal. Germany began a campaign of mass murder and forced resettlement, targeting Polish intellectuals, clergy, and Jews. The Soviet Union also carried out mass deportations and executions, notably the Katyn massacre of Polish officers in 1940. Poland lost about six million citizens during the war, including three million Polish Jews—the highest proportion of any country.
Frequently Asked Questions
Why did Germany invade Poland?
Hitler wanted to regain territories lost after World War I, particularly the Polish Corridor and Danzig, and to create living space (Lebensraum) for the German people in the east. He also intended to destroy the Polish state as a prelude to a larger war against the Soviet Union.
Why did the Soviet Union invade Poland?
The Soviet Union, under Joseph Stalin, invaded Poland on September 17, 1939, as part of the secret protocol of the Molotov-Ribbentrop Pact. Stalin wanted to regain territories lost after World War I and to create a buffer zone against Germany. The official justification was the collapse of the Polish state and the need to protect the Ukrainian and Belarusian minorities.
How long did the invasion of Poland last?
The German invasion lasted from September 1 to October 6, 1939, a total of 35 days. The Soviet invasion on September 17 accelerated the collapse. The last organized Polish resistance, the Battle of Kock, ended on October 6.
What was the Polish Corridor?
The Polish Corridor was a strip of land created by the Treaty of Versailles in 1919 that gave Poland access to the Baltic Sea, separating East Prussia from the rest of Germany. It was a major source of German resentment and one of Hitler’s stated reasons for the invasion.
Did Poland have any allies?
Poland had a military alliance with France (1921) and a guarantee of support from Britain (March 1939). However, neither country provided significant military assistance during the invasion. France and Britain declared war on Germany but did not launch a major offensive in the west.
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