Łódź  is Poland's second largest city. It is located in the centre of the country and serves as the capital of the Łódź Voivodeship.

During the Invasion of Poland the Polish forces of the Łódź Army of General Juliusz Rómmel defended Łódź against initial German attacks. However, the Wehrmacht captured the city on September 8. Despite plans for the city to become a Polish enclave, attached to the General Government, the Nazi hierarchy respected the wishes of the local governor of Reichstag Wartheland, Arthur Greiser, and of many of the ethnic Germans living in the city, and annexed it to the Reich in November 1939. The city received the new name of Litzmannstadt after the German general Karl Litzmann, who captured the city during World War I. Nevertheless, many Łódź Germans refused to sign Volksliste and become Volksdeutsche, instead being deported to the General Government.

Soon the Nazi authorities set up the Łódź Ghetto in the city and populated it with more than 200,000 Jews from the Łódź area. Only about 900 people survived the liquidation of the ghetto in August 1944. Several concentration camps and death camps arose in the city's vicinity for the non-Jewish inhabitants of the regions, among them the infamous Radogoszcz prison and several minor camps for the Roma people and for Polish children.

The invasion of Poland by Germany marked the start of World War II in Europe, as Poland's western allies, the United Kingdom, Australia and New Zealand, declared war on Germany on September 3, soon followed by France, South Africa and Canada, among others. The invasion began on September 1, 1939, one week after the signing of the Molotov-Ribbentrop Pact, and ended October 6, 1939, with Germany and the Soviet Union occupying the entirety of Poland. Although the United Kingdom and France declared war on Germany soon after Germany attacked Poland, very little direct military aid was provided.

Following a German-staged "Polish attack" on August 31, 1939, on September 1, German forces invaded Poland from the north, south, and west. Spread thin defending their long borders, the Polish armies were soon forced to withdraw eastward. After the mid-September Polish defeat in the Battle of the Bzura, the Germans gained an undisputed advantage. Polish forces then began a withdrawal southeast, following a plan that called for a long defense in the Romanian bridgehead area, where the Polish forces were to await an expected Allied counterattack and relief.

On September 17, 1939, the Soviet Red Army invaded the eastern regions of Poland in cooperation with Germany. The Soviets were carrying out their part of the secret appendix of the Molotov-Ribbentrop Pact, which divided Eastern Europe into Nazi and Soviet spheres of influence. Facing the second front, the Polish government decided the defense of the Romanian bridgehead was no longer feasible and ordered the evacuation of all troops to neutral Romania. By October 1, Germany and the Soviet Union completely overran Poland, although the Polish government never surrendered. In addition, Poland's remaining land and air forces were evacuated to neighboring Romania and Hungary. Many of the exiles subsequently joined the recreated Polish Army in allied France, French-mandated Syria, and the United Kingdom.

In the aftermath of the September Campaign, a resistance movement was formed. Poland's fighting forces continued to contribute to Allied military operations throughout World War II. Germany captured the Soviet-occupied areas of Poland when it invaded the Soviet Union on June 22, 1941, and lost the territory in 1944 to an advancing Red Army. Over the course of the war, Poland lost over 20% of its pre-war population under an occupation that marked the end of the Second Polish Republic.