What happened when Nazi Germany invaded Poland in September 1939?

Following the First World War, the Treaty of Versailles punished Germany through harsh military restrictions that limited its armed forces.

It also imposed heavy financial reparations and required the cession of territory, measures that stirred anger throughout the country.

Many Germans viewed the treaty as unjust, and public resentment created conditions that allowed radical political movements to grow.

Adolf Hitler, as leader of the National Socialist German Workers’ Party, gained support by promising to overturn the Versailles settlement and restore Germany’s status as a major power. 

During the early years of his rule, Hitler removed the treaty’s military limits. In 1935, he reintroduced conscription and began to expand the size of the Wehrmacht.

In March 1936, German troops marched into the Rhineland, which had been designated a arms-free zone after the First World War.

No foreign power intervened to stop the move. That lack of response encouraged Hitler to continue. 

In March 1938, German forces annexed Austria in a move referred to as the Anschluss.

Hitler claimed it was a reunion of the German people, and again, no country challenged the act.

Later in the year, Hitler turned to Czechoslovakia and demanded the Sudetenland, a region with a German-speaking population and solid defences.

British and French leaders who hoped to avoid war met with Hitler at Munich and agreed to his demands without consulting the Czechoslovak government. 

In early 1939, German troops marched into the rest of Czechoslovakia. For many in Europe, it confirmed that war with Germany had become increasingly likely. 

According to Nazi racial policy, Germany required new territory in the east to ensure the survival and growth of the Aryan race.

Hitler believed that Poland stood in the way of German expansion, and he intended to remove it as a sovereign state.

Nazi officials described this plan as the goal of Lebensraum, or “living space,” which would be seized at the expense of so-called inferior Slavic populations. 

In early 1939, Hitler began to demand the return of the Free City of Danzig, which had been placed under League of Nations authority after the First World War.

He also wanted a transport corridor to link Germany to East Prussia, which had been separated from the rest of the Reich.

However, Polish officials rejected these proposals, understanding that any concession would likely be followed by new demands. 

As tensions rose, Hitler secured a major diplomatic victory. On 23 August 1939, Germany signed the Molotov-Ribbentrop Pact with the Soviet Union.

Publicly, it promised that neither side would attack the other. In private, it contained a secret agreement that divided Poland and other parts of Eastern Europe into zones of German and Soviet influence.

Because the eastern front was secure, Hitler no longer faced the risk of a two-front war. 

Rather than negotiate further, Hitler completed plans for invasion. German propaganda increased claims of Polish mistreatment of ethnic Germans, which laid the groundwork for public support.

On 31 August, German agents staged a fake attack on a German radio station near the Polish border and blamed Polish saboteurs.

The staged attack on the Gleiwitz radio station involved SS operatives dressed in Polish uniforms, and it was part of a wider false-flag operation known as Operation Himmler, which gave the Nazi government a excuse to launch its assault the following day. 

Before sunrise on 1 September 1939, the German battleship Schleswig-Holstein opened fire on the Polish garrison at Westerplatte.

Simultaneously, German forces crossed into Poland from several directions, with attacks that advanced from East Prussia, Silesia, and Slovakia.

The German invasion force included approximately 1.5 million soldiers, 2,750 tanks, and 2,300 aircraft, while the Polish military fielded around 1 million troops with approximately 600 tanks and between 400 and 500 operational aircraft. 

In addition to German troops, an estimated 40,000 Slovak soldiers supported the southern advance, though only a portion participated directly in the fighting.

The Luftwaffe began bombing campaigns against major Polish cities, rail lines, and airfields in the first hours of the invasion.

One of the first Luftwaffe targets was the town of Wieluń, which had no military presence and suffered hundreds of civilian deaths in a bombing campaign that horrified observers, with estimates ranging from 200 to over 1,000. 

In many locations, German tank divisions avoided heavily defended areas and instead focused on quick moves toward important locations.

Polish military units were still in the process of mobilisation and found themselves isolated and encircled.

The German air force established immediate control of the skies, which destroyed most of the Polish air fleet before it could engage in combat. 

On 17 September, the Soviet Union invaded from the east. Stalin justified the move by claiming that the Polish state had collapsed and that Soviet troops had entered to protect ethnic minorities.

In reality, the invasion fulfilled the terms of the Molotov-Ribbentrop Pact. Soviet and German commanders planned their moves together, and both armies met near Brest-Litovsk to divide control of the country. 

Fighting in Warsaw continued until 27 September, when the city surrendered after a prolonged siege that involved sustained bombing raids and artillery bombardment.

Neighbouring cities such as Modlin and Lwów also fell in the final days of the campaign.

Polish resistance ended on 6 October, though scattered groups of soldiers and civilians continued to engage in small-scale operations.

President Ignacy Mościcki and Marshal Edward Rydz-Śmigły, Poland’s commander-in-chief, fled the country as the government collapsed, ultimately reaching Romania.

Prior to the invasion, Britain and France had assured Poland that they would defend its independence in the event of German aggression.

On 3 September 1939, Britain declared war on Germany, followed hours later by France.

While the declarations fulfilled treaty obligations, they came too late to prevent Poland’s collapse. 

In the weeks that followed, both countries prepared for another significant conflict.

Troops mobilised, air raid shelters were built, and the British government imposed blackout restrictions in cities to prevent bombing attacks.

Despite this activity, neither Britain nor France launched a major ground offensive.

Instead, the French army launched a minor advance into the Saar region on 7 September, known as the Saar Offensive, but the advance saw minimal engagement and was eventually withdrawn by mid-October without facing significant German resistance.

Along the Western Front, soldiers took up defensive positions, and the situation settled into a period of inaction that became known as the “Phoney War.” 

Outside Europe, the invasion of Poland drew widespread condemnation, though few governments took concrete steps.

The United States officially stayed neutral, but approved new rules that, by November 1939, allowed it to sell arms to belligerent countries on a cash-and-carry basis under strict conditions.

International organisations, including the League of Nations, failed to act on Poland's appeal for help. 

From Moscow, Soviet officials played down their attack, as they claimed that their actions were right because the Polish government had collapsed and the Red Army had entered to restore peace and order.

Western leaders criticised the Soviet move and focused their war efforts on Germany.

For the Polish people, the dual invasion caused terrible damage. The destruction of their state left them under foreign occupation, with no clear timeline for liberation. 

Following the defeat, Poland was partitioned in accordance with the Molotov-Ribbentrop Pact.

Germany annexed western and northern regions such as Silesia, Pomerania, and Poznań, making them part of the Reich.

The central portion became the General Government, a colonial-style administration headed by Hans Frank, which was intended to serve as a resource base and trial area for Nazi racial rules. 

Under German rule, Polish society faced immediate violence. Soldiers and SS units killed political leaders, thinkers, and clergy, and anyone suspected of opposing the regime.

During the first year of occupation, German forces executed over 60,000 members of the Polish intelligentsia in an operation known as the Intelligenzaktion.

Schools were shut down or repurposed, and instruction in Polish was restricted to the lowest levels of education, with most higher schools closed and replaced by only rudimentary vocational training.

In rural areas, farmland was seized and redistributed to ethnic Germans brought in as settlers.

Polish citizens faced restrictions on movement, speech restrictions, and severe limits on employment opportunities. 

Among those targeted most heavily were Poland’s Jews. German authorities established ghettos in cities such as Warsaw, Kraków, and Łódź, forcing Jewish families to live in districts that were overcrowded with unsanitary conditions and had sealed perimeters.

Starvation and disease spread rapidly, and over time, these ghettos became centres for forced deportations to extermination camps as part of the Final Solution. 

In the Soviet-occupied zone, Soviet officials arrested former Polish officers, government employees, landowners, and intellectuals.

Many were sent to labour camps in Siberia or Central Asia under charges of political disloyalty.

In April and May 1940, the NKVD executed more than 20,000 Polish prisoners in a series of mass killings known as the Katyn Massacre.

The mass graves were discovered by German forces in 1943, and the Soviet Union denied responsibility until 1990. 

Despite the occupation, in urban centres and rural communities, secret organisations distributed information, sabotaged German infrastructure, which prepared them for a future uprising.

The Home Army, which was loyal to the government-in-exile, became the largest resistance movement in occupied Europe, with membership reaching over 300,000 fighters at its peak.

Abroad, Polish military units reformed in France, Britain, and the Middle East, joining Allied forces in campaigns across multiple theatres of war.

Polish pilots famously took part in the Battle of Britain, with No. 303 Squadron, which achieved one of the highest kill rates in the RAF.

While Poland’s defeat had been swift, its people continued to fight for independence under the harshest conditions.