It is widely believed to this day that the Germans instigated the aerial bombing campaign against European and British cities, and thus “got what they deserved” in kind. Often cited are the German bombings of Warsaw and Rotterdam. These two instances of bombings are not in dispute, however, the context must be considered. The details are seldom discussed, and hence, little known. The rules of warfare must also be considered. The deliberate targeting of civilians was indeed a “war crime”. The Hague conventions tolerated the bombing of cities IF they were under military occupation (ie “defended”), which was the case with both Warsaw or Rotterdam.
The Hague Conventions for Land Warfare (which Germany was a signatory to) stated:
Article 25
The attack or bombardment of towns, villages, habitations or buildings which are not defended, is prohibited.
Article 26
The Commander of an attacking force, before commencing a bombardment, except in the case of an assault, should do all he can to warn the authorities.
Article 27
In sieges and bombardments all necessary steps should be taken to spare as far as possible edifices devoted to religion, art, science, and charity, hospitals, and places where the sick and wounded are collected, provided they are not used at the same time for military purposes.
The besieged should indicate these buildings or places by some particular and visible signs, which should previously be notified to the assailants.
http://avalon.law.yale.edu/19th_century/hague02.asp
https://justice4germans.wordpress.com/