Were the atomic attacks war crimes?

Was destroying cities with atomic bombs and killing their civilian populations a war crime?

The issue is complicated, but the attacks fell into a legal loophole because existing laws had not anticipated nuclear weapons.

The legal and moral question is complicated. Nuclear attacks had not been conceived when the relevant laws of war were written, leaving a major gap in the rules applied in 1945. No state has openly repeated such an attack since. The distinction between legality and morality matters because the legal loophole did not make the attacks morally unproblematic.