Invasion of Poland

Lore & Background

The invasion began on 1 September 1939 when Germany marched into Poland, preceded by the Danzig crisis and the Molotov–Ribbentrop Pact between Germany and the Soviet Union, signed on 23 August 1939 and approved by the Supreme Soviet one day before the invasion. On 17 September the Soviet Union invaded Eastern Poland, acting on a secret protocol of the pact. The campaign ended on 6 October with Germany and the Soviet Union dividing the whole of Poland under the German–Soviet Frontier Treaty, signed on 28 September 1939.

The aim was to disestablish Poland as a sovereign country, with its citizens destined for extermination. German and Slovak forces invaded from the north, south, and west after the Gleiwitz incident. Polish forces withdrew to the southeast only after the Soviet invasion on 17 September, planning a long defence of the Romanian Bridgehead while awaiting support from France and the United Kingdom.

Reader's Guide

The invasion of Poland is significant as the event that triggered World War II, demonstrating the effectiveness of Blitzkrieg tactics and the consequences of the Molotov–Ribbentrop Pact. It resulted in the complete partition of Poland between Germany and the Soviet Union, ending the Second Polish Republic. The invasion also led to the formation of the Polish Underground State and the Polish Armed Forces in the West, loyal to the government-in-exile. The failure of France and the United Kingdom to provide effective aid highlighted the limitations of their alliance with Poland. The campaign set a precedent for aggressive expansion and the disregard for international borders that characterized the war.

Did You Know?

• The invasion began on 1 September 1939, one day after the Supreme Soviet approved the Molotov–Ribbentrop Pact.
• The Soviet Union invaded Poland on 17 September 1939, acting on a secret protocol of the pact.
• Poland never formally surrendered, despite the defeat.

Diplomatic Manipulation and the Road to War

Hitler's ambition to dismantle Poland as a sovereign state was not inherited from earlier Weimar-era revisionists but was his own strategic vision, articulated as early as 1930 when he dismissed Poles and Czechs as unworthy of independent statehood. After seizing power on 30 January 1933, he initially pursued a policy of rapprochement with Warsaw, culminating in the 1934 German–Polish Non-Aggression Pact, while simultaneously working to sever Polish ties with France and pressuring Poland toward the Anti-Comintern Pact. He dangled the prospect of territorial gains in Ukraine and Belarus in exchange for a Polish war against the Soviet Union, a deal that would have reduced Poland to a German client state. The Free City of Danzig and the Polish Corridor became focal points of German nationalist grievance, with Hitler repeatedly invoking the plight of ethnic Germans as justification. The immediate trigger was the Molotov–Ribbentrop Pact, signed one week before the invasion and ratified by the Soviet Supreme Soviet just one day prior, whose secret protocol carved Poland into German and Soviet spheres. The Danzig crisis and the Gleiwitz border incident provided the final pretext for the 1 September attack.

The September Campaign on the Ground

On the morning of 1 September 1939, German and Slovak forces struck simultaneously from the north, south, and west, overwhelming Poland's forward defensive positions near the shared border. The Wehrmacht's rapid advance forced Polish units to fall back from their initial lines to more established defensive positions further east. The campaign's turning point came in mid-September with the Polish defeat at the Battle of the Bzura, after which the Germans held an undisputed operational advantage. Polish commanders regrouped in the southeast, preparing a prolonged defence of the Romanian Bridgehead while awaiting the promised military support from France and the United Kingdom. Although both powers declared war on Germany on 3 September under their alliance obligations, their tangible assistance proved minimal. France launched a small-scale Saar Offensive into German territory, but the Polish army was effectively destroyed before the British Expeditionary Force could even be transported to the continent. The campaign concluded on 6 October with the Polish defeat at the Battle of Kock, after which German and Soviet forces held complete control over Polish territory.

The Soviet Blow and the Collapse of Polish Strategy

The most devastating strategic blow to Poland did not come from the west but from the east. On 17 September, the Soviet Red Army crossed into Eastern Poland, occupying the territory beyond the Curzon Line that the secret protocol of the Molotov–Ribbentrop Pact had designated as the Soviet sphere of influence. This second-front invasion rendered the entire Polish defensive plan obsolete. With the Romanian Bridgehead now exposed to attack from both directions, the Polish government concluded that holding the position was no longer feasible and ordered an emergency evacuation of all remaining troops into neutral Romania. The dual pincer of German and Soviet forces sealed Poland's military fate on 6 October, following the final engagement at the Battle of Kock. The Second Polish Republic ceased to exist as a functioning state, yet Poland never issued a formal surrender. The German–Soviet Frontier Treaty, signed in the campaign's wake, formalized the division of the entire country between the two aggressors, leaving no Polish territory under Polish control.

Annexation, Occupation, and the Persistence of Polish Resistance

In the days immediately following the military defeat, the two occupiers imposed radically different administrative fates on the conquered lands. On 8 October, after a brief period of military administration, Germany directly annexed western Poland and the former Free City of Danzig, while placing the remaining central territory under the newly created General Government. The Soviet Union absorbed its seized eastern territories into the Byelorussian and Ukrainian constituent republics and immediately launched a campaign of Sovietization. Yet the destruction of the state did not extinguish Polish political life. Within the occupied territory, a collective of underground resistance organizations coalesced into the Polish Underground State, maintaining a shadow government in defiance of both occupiers. Meanwhile, military exiles who escaped to the west formed the Polish Armed Forces in the West, an armed force sworn to the Polish government-in-exile. These structures ensured that, despite the total military defeat and the partition of its land, Poland's claim to sovereignty persisted throughout the war.

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