German troops invaded Poland in the early morning of September 1, 1939. Two German army groups struck from the north and south in the direction of Warsaw, while the German Air Force began to bomb Polish cities. On the very first day of the war, the city of Danzig was annexed to Germany.

Although Poland declared a general mobilization on September 1, 1939, its situation quickly deteriorated. During the first two weeks of the war, the Germans routed the Polish Army, seizing major political and economic centres in the west of Poland despite a few continuing pockets of resistance. On September 17, 1939, the Polish Army ceased to exist. The Polish government abandoned the country and fled to Romania in the night of September 17–18. Indeed, many Polish officers had obtained Romanian visas and left Poland even before the start of the war.

The defence of Warsaw continued until its garrison, depleted of manpower and materials, capitulated on September 28, 1939. A large number of Polish soldiers were taken captive.

The success of the Wehrmacht in Poland was due to the political mistakes of its leadership, which, taking a hard anti-­Soviet line, aimed exclusively at making an alliance with France and Great Britain. These two countries, however, left Poland to its fate after the start of the German invasion.

The Polish government-in-exile formed in Paris on September 30, 1939, and subsequently moved to London. This government was supported by the French and British allies.

In October 1939, the Germans established the General Governorate for the Occupied Polish Region, which was initially divided into four administrative districts: Cracow, Warsaw, Lublin and Radom. Cracow was chosen as the capital of the General Governorate and the site of the German occupation administration.