Hiroshima, since it had been one of the two targets for a nuclear attack. It had thought to been a strategic military target but many civilians had died in the process and leveled the city with only one of the nuclear devices.
Tokyo did not suffer a nuclear attack but the city was also badly damaged with multiple firebombing from American planes. This as well took the lives of civilians throughout the war.
Overall, I'd say Hiroshima had crossed a moral threshold.