The actions like the bombing of Dresden or the dropping of the atomic bombs in Hiroshima and Nagasaki were not treated as war crimes mainly because of two reasons, one being that they were attacks on the aggressor countries, and the other being that the courts that were charging for war crimes were pretty much in the hands of the winning side.
I personally do not agree with that, as in those attacks the main victims were civilians that had nothing to do with the war, so by any logic that is a war crime. Also, the effects of these attacks were long lasting, and even nowadays there's consequences from some them as they have a long lasting radiation. Any attack on innocent people with deadly weapons is a war crime, thus those should have been put in the same ''bucket''.