The Polish September Campaign — known also as the "Polish-German War of 1939", in Poland sometimes as the "1939 Defensive War" (Wojna obronna 1939 roku), in Germany as the "Poland Campaign", and codenamed Fall Weiss ("Case White") by the German General Staff — was the World War II invasion of Poland by military forces of Nazi Germany and the Soviet Union and by 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 and France, declared war on Germany on September 3. The campaign began on September 1, 1939, one week after the signing of the secret Molotov-Ribbentrop Pact, and ended on October 6, 1939, with Germany and the Soviet Union occupying the entirety of Poland. None of the major participants — Germany, the Soviet Union, Poland or the Western Allies — expected that this German invasion of Poland would lead to a war surpassing World War I in scale and cost.
Following a spurious, German-staged "Polish attack" on 1 September 1939, German forces invaded Poland from the north, south, and west. Spread thin defending their long borders, the Polish armies were soon forced to withdraw east. After the mid-September Polish defeat in the Battle of the Bzura, the Germans gained an undisputed advantage. Polish forces then began a withdrawal south-east, following a plan that called for a long defence in the Romanian bridgehead area where Polish forces were to await an expected Western Allies 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. This invasion secured Hitler's right flank and allowed him to concentrate on attacking Poland. Because of this unexpected Soviet aggression, the Polish government decided that the defence of the Romanian bridgehead was no longer feasible and ordered the evacuation of all troops to neutral Romania. By 1 October, Germany and the Soviet Union had completely overrun Poland. The Polish government (which never surrendered) evacuated together with many of its remaining land and air forces 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.