(obviously) Horoshima/Nagasaki would obviously be war crimes today

The same man was the President of the USA when nuclear bombs were dropped on Japan and when the Geneva Convention of 1949 was written. There was certainly discussion at the time about whether the Allied bombing campaigns had been in accordance with the laws of war, but the stance of the US government was that they were, and this stance was apparently not incompatible with the Geneva Convention of 1949.