The first bombing of London was a mistake, but it would trigger policy on both sides which would result in the loss of millions of civilians. At the beginning of the war, the mass bombing of cities was a great fear, and both sides avoided it, although they prepared for it. The built air raid shelters, issued gas masks and prepared defenses. The initial policies held that aircraft would only bomb military targets.
However the Nazis did bomb Warsaw, after the Poles declared that they would defend the city to the last. The city was surrendered after all utilities were destroyed. The British bombers, after finding their planes were more readily shot down during the day, began bombing at night. Navigation issues lead to bombs missing military sites and causing civilian casualties.
Another Nazi attack against a city was that of Rotterdam in Holland. This bombing took place after the surrender was too slow.
However, full scale bombing on both sides did not take place until after an accidental bombing in London. Both sides had escalated bombing against targets closer and closer to population centers.
Early in August of 1940 that Luftwaffe was commanded to go after the British RAF. This would take German planes over population centers. Hitler ordered there there should be no terror bombing. This was a period of intense air battles. Germans had strength in numbers, while the British were closer to home and able to stay over the battle area longer. The German's changed tactics by having bombers attack at night. They were hoping to stretch the British air defenses. However, the germans had problems with night navigation. Some of the planes became confused. At least one dropped its bombs anyway, hitting civilians in London. Nine civilians were killed.
The British retaliated, and then things escalated from there. Berlin was an immediate target for the RAF. Before the end of the war there were fire bombings of major population centers.