"The German ultimatum ordering the Dutch commander of Rotterdam to cease fire was delivered to him at 10:30h on 14 May 1940. At 13:22h, German bombers set the whole inner city of Rotterdam ablaze, killing 814 of its inhabitants." The photo was taken after the removal of all debris. [Source, Via Wikipedia Commons: http://www.archives.gov/research_room/arc/ ARC Identifier: 535916; U.S. Defense Visual Information Center photo HD-SN-99-02993]
While this was not the first bombing of a city by a long way, it was in many ways the precedent that unleashed the dreadful bomber war where millions of civilians (engaged in war work or not) as well as the military were killed, disabled or made homeless in the pursuit of destroying a nations will to fight.
This was the 'wind' that 'Bomber' Harris referred to when he stated that, quoting the Bible:
"The Nazis entered this war under the rather childish delusion that they were going to bomb everybody else, and nobody was going to bomb them. At Rotterdam, London, Warsaw, and half a hundred other places, they put that rather naive theory into operation. They sowed the wind, and now, they are going to reap the whirlwind."
Today, in Rotterdam, a forthcoming exhibition "The Attack, May 1940" is centred around a post-W.W.II Spanish CASA 2111 bomber, descendant of the Luftwaffe's bombers of May 1940, specifically the Heinkel He 111. This example is on loan from a German collection.
For the reasons above, this is one of the most important exhibitions about W.W.II.
Exhibition details here: http://museumrotterdam.nl/tentoonstellingen/de-aanval
Translated details here: http://translate.google.com.au/translate?hl=en&sl=nl&u=http://museumrotterdam.nl/tentoonstellingen/de-aanval&prev=search