The Invasion of Poland, 1939 (in Poland also "the September Campaign," "Kampania wrześniowa," and "the 1939 Defensive War," "Wojna obronna 1939 roku"; in Germany, "the Poland Campaign," "Polenfeldzug," codenamed "Fall Weiss," "Case White," by the German General Staff, and sometimes called "the Polish-German War of 1939"), which precipitated World War II, was carried out by Nazi Germany, the Soviet Union and a small German-allied Slovak contingent.

The invasion of Poland 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 (see Phoney War and Western betrayal).

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. Baliszewski, Most honoru

On September 17 1939, the Soviet Red Army invaded the eastern regions of Poland in cooperation with Germany. Fischer 1999–2000 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.