Hm, difficult but interesting question..
One of the problems after WW2 was that the rules of what was war crimes or not hadn't been clearly defined yet..
The fourth Geneva Convention which handles the protection of civilians in time of war wasn't signed until August 1949.
If a bombing of a city on that scale had happened today, it would have been classified as a war crime, but back then the cities was regarded as military targets since the breaking of the German morale was considered a deceicive(?) point.
Civlian industrial workers was also considered military targets.
I seem to remember that about 600,000 German civilians were killed by allied bombing of german cities, about 45,000 were killed in just one night alone in Hamburg July 1943....
Here's an interesting article about this topic:
I haven't seen this site before, so I can't guarantee for it's credibility.. http://www.fpp.co.uk/online/00/02/Globe180200.html
and another link: http://www.guardian.co.uk/elsewhere/...068437,00.html