Following several German-staged incidents (like the Gleiwitz incident, a part of Operation Himmler), which German propaganda used as an excuse to claim that German forces were acting in self-defence, the first regular act of war took place on 1 September 1939, at 04:40, when the Luftwaffe attacked the Polish town of Wieluń, destroying 75% of the city and killing close to 1,200 people, most of them civilians. This invasion subsequently began World War II. Five minutes later, the old German pre-dreadnought battleship Schleswig-Holstein opened fire on the Polish military transit depot at Westerplatte in the Free City of Danzig on the Baltic Sea. At 08:00, German troops—still without a formal declaration of war issued—attacked near the Polish town of Mokra. The Battle of the Border had begun. Later that day, the Germans attacked on Poland's western, southern and northern borders, while German aircraft began raids on Polish cities. The main axis of attack led eastwards from Germany proper through the western Polish border. Supporting attacks came from East Prussia in the north, and a co-operative German-Slovak tertiary attack by units (Field Army "Bernolák") from German-allied Slovakia in the south. All three assaults converged on the Polish capital of Warsaw.
The Soviet Union will soon ally with Germany (yes, you read that right), and also invade Poland from the east. Germany and the U.S.S.R. will divide Poland among themselves on October 6.