The LAW OF WAR, Dimwit. It was used in the South during CWI and has ALWAYS worked.
The USA, Germany, and Britain were signatories of a number of treaties and conventions that regulate proper behavior in an armed conflict. Deliberately attacking civilians was wrong for the Germans to do, and also wrong for the Allies to do.
The best that is sometimes said about all these bombings on both sides is that they were technically legitimate at the time, but the Geneva Convention of 1949 condemns it. An of course look at it as you may, there were massive avoidable civilian casualties.