"civilian targets were not acceptable" To whom? You need to know some history. The reason why Tokyo was not targeted is because it had already been razed. Most homes in Japan were flimsy wooden structures. The US airforce used incendiaries. Japan had no concept of "civilian/military". Housewives were being trained to use any weapon they could in order to kill American forces. There is film of this training. Britain's Royal Airforce was bombing German cities in retaliation for Germany's attacks on population centres. Civilians were at risk as soon as aircraft could carry bombloads sufficient to do real damage.
Apart from revenge, practically the idea was to hinder war production. Dead people don't make munitions and weapons. Bombing was also intended to demoralise and terrify civilians. Bombing factories, staffed by civilians, hindered war production of course.
Bombing in WW2, especially early in the war, was incredibly inaccurate. Even if the target was military, civilians often suffered. War is hell. Human nature is no better now than in the days of the worst excesses of ancient cultures. All that is changed is the means of causing destruction and suffering.