Wikipedia: "researchers have asserted that not all of the communications infrastructure, such as the bridges, was targeted, nor were the extensive industrial areas outside the city centre. Critics of the bombing argue that Dresden was a cultural landmark of little or no military significance, and that the attacks were indiscriminate area bombing and not proportionate to the commensurate military gains."
Winners of wars have historically written history and decided how wartime actions are viewed. Hence Allied forces as the victors got to choose who would stand trial for war crimes.
In modern wars (practically starting with the Vietnam war), the growing power of media and plummeting costs of acquiring and transmitting photos and videos have given even the losers the power to subvert narratives put out by victors, hence the necessity of even powerful forces to take additional precautions.
The comments in this Quora article on this very subject[0] make a good case that the atomic bombings weren't war crimes. As much as I disagree with the atomic bombings on moral and humanitarian grounds, and have a general knee-jerk bias against American militarism in general, it seems the US didn't violate the rules of war which were understood at the time and that Hiroshima and Nagasaki were legitimate military targets. The atomic bombings were acts of warfare, but not war crimes.
Of course, it's worth mentioning that the #1 answer in that article is "Because the US won," which is also true. Even if the atomic bombings had been war crimes, who's going to put the world's only (at the time) nuclear superpower on trial for them?
It is not that simple, consider the WWII bombing of Dresden (which is defendable IMHO), for example...
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