Prelude to the campaign

The Nazi Party, led by Adolf Hitler, took power in Germany in 1933. Hitler at first pursued a policy of rapprochement with Poland, culminating in the German-Polish Non-Aggression Pact of 1934. However, following Germany's annexation of Austria in 1938, and most of Czechoslovakia in 1939, under the continued Allied policy of appeasement, the Nazi regime turned its attention to Poland. Of special concern to Germany was the Free City of Danzig and the fact that German exclave of East Prussia was separated from mainland by little-known territory called the Polish Corridor, a narrow strip of land allowing Poland access to the Baltic Sea. The Free City of Danzig was a continual annoyance for the Germans. Having been a "lost" territory after Germany's defeat in World War I, Hitler roused German nationalism by promising to "liberate" the Germans still living there. In early 1939, Hitler issued secret orders to prepare for the "solution of the Polish problem by military means", and the German government intensified demands for the annexation of the Free City of Danzig, as well as for construction of an extra-territorial road through the Polish Corridor connecting East Prussia with the rest of Germany. The Fall Weiss plan was ready by April 3.

Hitler and most of his advisors expected the Polish government to yield to these demands. However, the Polish government rejected them and was backed on March 30 by guarantees from Britain and France, now concerned with German expansionism. Hitler, undaunted, believed that Britain and France could be persuaded from entering the coming war. On 28 April, Germany withdrew from both the German-Polish Non-Aggression Pact of 1934 and the London Naval Agreement of 1935. Meanwhile Poland, ironically, itself tried to benefit from German expansionism by annexing the Czechoslovakian town of Cieszyn (Český Těšín) in the aftermath of the Munich agreement.

Between 1919 and 1939, the basic goal of British foreign policy had been to prevent another world war by a mixture of "carrot and stick". The "stick" in this case was the "guarantee" of March 1939, which was intended to prevent Germany from attacking either Poland or Romania. At the same time, Prime Minister Neville Chamberlain and his Foreign Secretary Lord Halifax hoped to offer Hitler a "carrot" in the form of another deal similar to the Munich Agreement, which would see the Free City of Danzig and the Polish Corridor returned to Germany in exchange for a promise to leave the rest of Poland alone.

This declaration was further amended in April, when Poland's minister of foreign affairs Colonel Józef Beck met with Neville Chamberlain and Lord Halifax. On August 25 the Polish-British Common Defence Pact was signed as an annex to the Franco-Polish Military Alliance, committing Britain to the defence of Polish independence.

Meanwhile, secret German-Soviet talks were held in Moscow, which resulted in signing of the Molotov-Ribbentrop Pact on August 22, by which Hitler intended to neutralize the possibility of the Soviet Union resisting an invasion of its western neighbour. In a secret protocol of this pact, the Germans and the Soviets agreed that Poland should be divided between them, with the western third of the country going to Germany and the eastern two-thirds to the Soviet Union. Although Western Allies' intelligence had uncovered the secret appendix concerning Poland, this information was not shared with the Polish government.

The German assault was originally scheduled to begin at 0400 on 26 August. However, on 25 August, Britain announced that its guarantee of Polish independence had been formalized by an alliance between the two countries. Hitler wavered and postponed his attack until 1 September, while trying on 26th of August to dissuade the British and the French from interfering in the eventual conflict. The negotiations convinced Hitler that there was little chance the Western Allies would declare war on Germany, and even if they did, due to 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 number of cross-border raids and sabotages by German Abwehr units, border skirmishes and increased overflights by high-altitude reconnaissance aircraft, signalled that war was imminent.

On 29 August, Germany issued Poland a final ultimatum demanding the Polish Corridor, which the Poles refused to consider. German Foreign Minister Joachim von Ribbentrop declared negotiations with Poland to be at an end, and Polish forces braced for war. On 30 August, the Polish Navy sent its destroyer flotilla to Britain to avoid destruction by the overwhelming German Kriegsmarine on the small Baltic Sea. On the same day, Polish Marshal Rydz-Śmigly announced the war mobilization of Polish troops, but was pressured into revoking the order by the French, who still hoped for a diplomatic settlement, and failed to realize that the Germans were fully mobilized and concentrated at the Polish border. On 31 August 1939, Hitler ordered hostilities against Poland to start at 4:45 the next morning. Poland managed to mobilise only 70% of its planned forces, and many units were still forming or moving to their designated frontline positions.