The Invasion of Poland in 1939 precipitated World War II. It was carried out by Nazi Germany, the Soviet Union, and a small Slovak contingent. In Poland the invasion is also known as the September Campaign (Kampania wrześniowa) or the 1939 Defensive War (Wojna obronna 1939 roku). In Germany it is sometimes referred to as the Poland Campaign (Polenfeldzug) or the Polish-German War of 1939. For the German General Staff, it was codenamed Unternehmen Fall Weiss, or Case White.

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,[8] declared war on Germany on September 3, soon followed by France, South Africa and Canada, among others. The invasion began on 1 September 1939, one week after the signing of the Molotov–Ribbentrop Pact, and ended 6 October 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 31 August 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.[9]

On 17 September 1939, the Soviet Red Army invaded the eastern regions of Poland in cooperation with Germany. [10][11]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.[12] Facing the second front, the Polish government decided the defense of the Romanian bridgehead was no longer feasible and ordered the emergency evacuation of all troops to neutral Romania.[13] By 1 October, 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 invasion, a resistance movement was formed. Poland's fighting forces continued to contribute to Allied military operations throughout World War II. On 8 October, Nazi Germany annexed the western areas of pre-war Poland and established the Generalgouvernement from the remains of her gains. The Soviet Union temporarily lost her gains due to the Nazi German invasion of June 1941, but permanently re-annexed them after regaining them in mid-1944. 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.