The invasion of Poland (September campaign, Polish: Kampania wrześniowa; 1939 defensive war, Polish: Wojna obronna 1939 roku; Poland campaign, German: Überfall auf Polen) marked the beginning of World War II. The German invasion began on 1 September 1939, one week after the signing of the Molotov–Ribbentrop Pact between Germany and the Soviet Union, and one day after the Supreme Soviet of the Soviet Union had approved the pact. The Soviets invaded Poland on 17 September. The campaign ended on 6 October with Germany and the Soviet Union dividing and annexing the whole of Poland under the terms of the German–Soviet Frontier Treaty.
German forces invaded Poland from the north, south, and west the morning after the Gleiwitz incident. Slovak military forces advanced alongside the Germans in northern Slovakia. As the Wehrmacht advanced, Polish forces withdrew from their forward bases of operation close to the Germany–Poland border to more established defense lines to the east. After the mid-September Polish defeat in the Battle of the Bzura, the Germans gained an undisputed advantage. Polish forces then withdrew to the southeast where they prepared for a long defence of the Romanian Bridgehead and awaited expected support and relief from France and the United Kingdom. Those two countries had pacts with Poland and had declared war on Germany on 3 September; in the end their aid to Poland was very limited, however France invaded a small part of Germany in the Saar Offensive.
On 17 September, the Soviet Red Army invaded Eastern Poland, the territory beyond the Curzon Line that fell into the Soviet “sphere of influence” according to the secret protocol of the Molotov–Ribbentrop Pact; this rendered the Polish plan of defence obsolete. Facing a second front, the Polish government concluded the defence of the Romanian Bridgehead was no longer feasible and ordered an emergency evacuation of all troops to neutral Romania. On 6 October, following the Polish defeat at the Battle of Kock, German and Soviet forces gained full control over Poland. The success of the invasion marked the end of the Second Polish Republic, though Poland never formally surrendered.
On 8 October, after an initial period of military administration, Germany directly annexed western Poland and the former Free City of Danzig and placed the remaining block of territory under the administration of the newly established General Government. The Soviet Union incorporated its newly acquired areas into its constituent Byelorussian and Ukrainian republics, and immediately started a campaign of Sovietization. In the aftermath of the invasion, a collective of underground resistance organizations formed the Polish Underground State within the territory of the former Polish state. Many of the military exiles that managed to escape Poland subsequently joined the Polish Armed Forces in the West, an armed force loyal to the Polish government-in-exile.
On 30 January 1933, the National Socialist German Workers’ Party, under its leader Adolf Hitler, came to power in Germany. While the Weimar Republic had long sought to annex territories belonging to Poland, it was Hitler’s own idea and not a realization of Weimar plans to invade and partition Poland, annex Bohemia and Austria, and create satellite or puppet states economically subordinate to Germany. As part of this long-term policy, Hitler at first pursued a policy of rapprochement with Poland, trying to improve opinion in Germany, culminating in the German–Polish Non-Aggression Pact of 1934. Earlier, Hitler’s foreign policy worked to weaken ties between Poland and France, and attempted to manoeuvre Poland into the Anti-Comintern Pact, forming a cooperative front against the Soviet Union. Poland would be granted territory to its northeast in Ukraine and Belarus if it agreed to wage war against the Soviet Union, but the concessions the Poles were expected to make meant that their homeland would become largely dependent on Germany, functioning as little more than a client state. The Poles feared that their independence would eventually be threatened altogether; historically Hitler had already denounced the right of Poland to independence in 1930, writing that Poles and Czechs are a “rabble not worth a penny more than the inhabitants of Sudan or India. How can they demand the rights of independent states?”
The population of the Free City of Danzig was strongly in favour of annexation by Germany, as were many of the ethnic German inhabitants of the Polish territory that separated the German exclave of East Prussia from the rest of the Reich. The Polish Corridor constituted land long disputed by Poland and Germany, and inhabited by a Polish majority. The Corridor had become a part of Poland after the Treaty of Versailles. Many Germans also wanted the urban port city of Danzig and its environs (comprising the Free City of Danzig) to be reincorporated into Germany. Danzig city had a German majority, and had been separated from Germany after Versailles and made into the nominally independent Free City. Hitler sought to use this as casus belli, a reason for war, reverse the post-1918 territorial losses, and on many occasions had appealed to German nationalism, promising to “liberate” the German minority still in the Corridor, as well as Danzig.
The invasion was referred to by Germany as the 1939 Defensive War (Verteidigungskrieg) since Hitler proclaimed that Poland had attacked Germany and that “Germans in Poland are persecuted with a bloody terror and are driven from their homes. The series of border violations, which are unbearable to a great power, prove that the Poles no longer are willing to respect the German frontier.”
Poland participated with Germany in the partition of Czechoslovakia that followed the Munich Agreement, although they were not part of the agreement. It coerced Czechoslovakia to surrender the region of Český Těšín by issuing an ultimatum to that effect on 30 September 1938, which was accepted by Czechoslovakia on 1 October. This region had a Polish majority and had been disputed between Czechoslovakia and Poland in the aftermath of World War I. The Polish annexation of Slovak territory (several villages in the regions of Čadca, Orava and Spiš) later served as the justification for the Slovak state to join the German invasion.
By 1937, Germany began to increase its demands for Danzig, while proposing that an extraterritorial roadway, part of the Reichsautobahn system, be built in order to connect East Prussia with Germany proper, running through the Polish Corridor. Poland rejected this proposal, fearing that after accepting these demands, it would become increasingly subject to the will of Germany and eventually lose its independence as the Czechs had. Polish leaders also distrusted Hitler. The British were also wary of Germany’s increasing strength and assertiveness threatening its balance of power strategy. On 31 March 1939, Poland formed a military alliance with the United Kingdom and with France, believing that Polish independence and territorial integrity would be defended with their support if it were to be threatened by Germany. On the other hand, British Prime Minister Neville Chamberlain and his Foreign Secretary, Lord Halifax, still hoped to strike a deal with Hitler regarding Danzig (and possibly the Polish Corridor). Chamberlain and his supporters believed war could be avoided and hoped Germany would agree to leave the rest of Poland alone. German hegemony over Central Europe was also at stake. In private, Hitler said in May that Danzig was not the important issue to him, but pursuit of Lebensraum for Germany.
Breakdown of talks
With tensions mounting, Germany turned to aggressive diplomacy. On 28 April 1939, Hitler unilaterally withdrew from both the German-Polish Non-Aggression Pact of 1934 and the Anglo-German Naval Agreement of 1935. Talks over Danzig and the Corridor broke down and months passed without diplomatic interaction between Germany and Poland. During this interim period, the Germans learned that France and Britain had failed to secure an alliance with the Soviet Union against Germany, and that the Soviet Union was interested in an alliance with Germany against Poland. Hitler had already issued orders to prepare for a possible “solution of the Polish problem by military means” through the Case White scenario.
In May 1939, in a statement to his generals while they were in the midst of planning the invasion of Poland, Hitler made it clear that the invasion would not come without resistance as it had in Czechoslovakia:
With minor exceptions German national unification has been achieved. Further successes cannot be achieved without bloodshed. Poland will always be on the side of our adversaries… Danzig is not the objective. It is a matter of expanding our living space in the east, of making our food supply secure, and solving the problem of the Baltic states. To provide sufficient food you must have sparsely settled areas. There is therefore no question of sparing Poland, and the decision remains to attack Poland at the first opportunity. We cannot expect a repetition of Czechoslovakia. There will be fighting.
On 22 August, just over a week before the onset of war, Hitler delivered a speech to his military commanders at the Obersalzberg:
The object of the war is … physically to destroy the enemy. That is why I have prepared, for the moment only in the East, my ‘Death’s Head’ formations with orders to kill without pity or mercy all men, women, and children of Polish descent or language. Only in this way can we obtain the living space we need. 
With the surprise signing of the Molotov–Ribbentrop Pact on 23 August, the result of secret Nazi–Soviet talks held in Moscow, Germany neutralized the possibility of Soviet opposition to a campaign against Poland and war became imminent. In fact, the Soviets agreed not to aid France or the UK in the event of their going to war with Germany over Poland and, in a secret protocol of the pact, the Germans and the Soviets agreed to divide Eastern Europe, including Poland, into two spheres of influence; the western one-third of the country was to go to Germany and the eastern two-thirds to the Soviet Union.
The German assault was originally scheduled to begin at 04:00 on 26 August. However, on 25 August, the Polish-British Common Defence Pact was signed as an annex to the Franco-Polish alliance (1921). In this accord, Britain committed itself to the defence of Poland, guaranteeing to preserve Polish independence. At the same time, the British and the Poles were hinting to Berlin that they were willing to resume discussions—not at all how Hitler hoped to frame the conflict. Thus, he wavered and postponed his attack until 1 September, managing to in effect halt the entire invasion “in mid-leap”.Planned and actual divisions of Poland, according to the Molotov–Ribbentrop Pact, with later adjustments
However, there was one exception: on the night of 25–26 August, a German sabotage group which had not heard anything about a delay of the invasion made an attack on the Jablunkov Pass and Mosty railway station in Silesia. On the morning of 26 August, this group was repelled by Polish troops. The German side described all this as an incident “caused by an insane individual” (see Jabłonków incident).
On 26 August, Hitler tried to dissuade the British and the French from interfering in the upcoming conflict, even pledging that the Wehrmacht forces would be made available to Britain’s empire in the future. The negotiations convinced Hitler that there was little chance the Western Allies would declare war on Germany, and even if they did, because of the lack of “territorial guarantees” to Poland, they would be willing to negotiate a compromise favourable to Germany after its conquest of Poland. Meanwhile, the increased number of overflights by high-altitude reconnaissance aircraft and cross-border troop movements signaled that war was imminent.The map shows the beginning of World War II in September 1939 in a European context.
On 29 August, prompted by the British, Germany issued one last diplomatic offer, with Fall Weiss yet to be rescheduled. That evening, the German government responded in a communication that it aimed not only for the restoration of Danzig but also the Polish Corridor (which had not previously been part of Hitler’s demands) in addition to the safeguarding of the German minority in Poland. It said that they were willing to commence negotiations, but indicated that a Polish representative with the power to sign an agreement had to arrive in Berlin the next day while in the meantime it would draw up a set of proposals. The British Cabinet was pleased that negotiations had been agreed to but, mindful of how Emil Hácha had been forced to sign his country away under similar circumstances just months earlier, regarded the requirement for an immediate arrival of a Polish representative with full signing powers as an unacceptable ultimatum. On the night of 30/31 August, German Foreign Minister Joachim von Ribbentrop read a 16-point German proposal to the British ambassador. When the ambassador requested a copy of the proposals for transmission to the Polish government, Ribbentrop refused, on the grounds that the requested Polish representative had failed to arrive by midnight. When Polish Ambassador Lipski went to see Ribbentrop later on 31 August to indicate that Poland was favorably disposed to negotiations, he announced that he did not have the full power to sign, and Ribbentrop dismissed him. It was then broadcast that Poland had rejected Germany’s offer, and negotiations with Poland came to an end. Hitler issued orders for the invasion to commence soon afterwards.
On 29 August, Polish Minister of Foreign Affairs, Józef Beck ordered military mobilization, but under the pressure from Great Britain and France, the mobilization was cancelled. When the final mobilization started, it added to the confusion.
On 30 August, the Polish Navy sent its destroyer flotilla to Britain, executing the Peking Plan. On the same day, Marshal of Poland Edward Rydz-Śmigły announced the mobilization of Polish troops. However, he was pressured into revoking the order by the French, who apparently still hoped for a diplomatic settlement, failing to realize that the Germans were fully mobilized and concentrated at the Polish border. During the night of 31 August, the Gleiwitz incident, a false flag attack on the radio station, was staged near the border city of Gleiwitz in Upper Silesia by German units posing as Polish troops, as part of the wider Operation Himmler. On 31 August 1939, Hitler ordered hostilities against Poland to start at 4:45 the next morning. However, partly because of the earlier stoppage, Poland finally managed to mobilize only about 70% of its planned forces (only about 900,000 of 1,350,000 soldiers planned to mobilize in first order), and because of that many units were still forming or moving to their designated frontline positions. The late mobilization reduced combat capability of Polish Army by about 1/3.