The International Court of Justice gave an advisory opinion in July 1996 on the Legality of the Threat Or Use Of Nuclear Weapons. The court ruled that "[t]here is in neither customary nor international law any comprehensive and universal prohibition of the threat or use of nuclear weapons." However, by a split vote, it also found that "[t]he threat or use of nuclear weapons would generally be contrary to the rules of international law applicable in armed conflict." The Court stated that it could not definitively conclude whether the threat or use of nuclear weapons would be lawful or unlawful in an extreme circumstance of self-defense, in which the very survival of the state would be at stake.
The same judgement still applies to strategic bombing, as there are no specific rules on aerial bombardment in the Geneva Conventions. The principles for judging legality are military necessity, distinction, and proportionality. The GC do put the obligation on the defenders not to deliberately place troops, weapons factories, etc. in densely populated areas.
Thus, I think Dresden would still be legal today.