I think you may be starting on the wrong foot - mass bombings of civilians was never widely accepted as an ok thing to do, people grappled with the morality and consequences of it from the beginning. For example, areal bombardments during the Spanish Civil War and the Japanese invasion of Manchuria drew wide condemnation. Even during the Second World War the Germans accused the Allies of war crimes, arguing that killing a civilian with a bomb from the air was no different than shooting a civilian on the ground. Many of the people in the Allied nations were conflicted with the bombings, but there was not much else that could be done from their perspective. Effective AA weapons forced bombers to perform high altitude night bombings which were so inaccurate they would just as often miss the entire city they were aiming for.

The original idea was that these bombing campaigns would break the morale of the civilians, but repeated bombings and wars showed this is not the case. The Germans, Japanese, and English all held their morale in the face of devastation. At best it can destroy some of a nations industrial capabilities. Nations who were bombed learned from this and either moved their industry to inaccessible regions like the Soviets did with the Urals, or made a distributed system like the Germans did. It did have some effect, but not nearly the desired effect.

The historiography is pretty deep and has been reassessed many times. As a starting point The Bombers and the Bombed by Richard Overy is an excellent look at the morality of the bombing campaign in the European theater of WW2. Dr. Gary Sheldon from Princeton is in the middle of a lecture tour "On the Transnational Destruction of Cities: What Japan and the U.S. Learned from the Bombing of Britain and Germany in World War II", so if you can catch him he gave an excellent presentation and is where I draw many of my notes from.

so here is what i can say from my limited knowledge. WW II was an example of total war. that is a fancy way of saying everything under your enemies control is a fair target. It is the type of warfare that operates outside of the normal conventions of war that had existed up to that point. In total war things like civilian moral and food production are part of the war effort, therefor a ligament target. Also one must keep in mind the Geneva Convention predates air based warfare so there was a slight grey area in it when it came to implementation in the age of flight. Probably the most notable bombing of civilian targets in Europe that comes to my mind happened towards the end of WW II in Dresden which the debate runs even to today on whether this raid was justified or a war crime.

It seems to me that the moment has come when the question of bombing of German cities simply for the sake of increasing the terror, though under other pretexts, should be reviewed. Otherwise we shall come into control of an utterly ruined land… The destruction of Dresden remains a serious query against the conduct of Allied bombing. I am of the opinion that military objectives must henceforward be more strictly studied in our own interests than that of the enemy. The Foreign Secretary has spoken to me on this subject, and I feel the need for more precise concentration upon military objectives such as oil and communications behind the immediate battle-zone, rather than on mere acts of terror and wanton destruction, however impressive

That was the content of a memo sent by Churchill 2 days after the bombing of Dresden to the British Chiefs of Staff and the Chief of the Air Staff.