From Wikipedia

The Bombing of Germany

Strategic bombing during World War II is a term which refers to all aerial bombardment of a strategic nature between 1939 and 1945 involving any nations engaged in World War II. This includes the bombing of military forces, railways, harbors, cities (civilian areas), and industrial areas.

In 1939, Germany invaded Poland and the Luftwaffe (German air force) began providing tactical support to the German Army. The Luftwaffe also began eliminating strategic objectives and bombing cities in Poland. France and the United Kingdom declared war on Germany and the UK's Royal Air Force began attacking German warships along the German coast with the North Sea. Meanwhile, the German bombing of Poland became an indiscriminate and unrestricted aerial bombardment campaign.