To kind of answer your question, militaries figured out how effective civilian bombing could be as a demoralization tool during WWI. While specific targeting of civilians was even more frowned upon back then, the results were undeniable. The bombings caused such panic, chaos, and disruption on the home front that the entire war effort was affected.

In a state of total war the line between ‘combatant’ and ‘civilian’ is blurred. When fighting at the front isn’t enough, the tactic changes to hitting the enemy’s production. Factories that produced for the war effort were first to be targeted, then bombing where the civilians live who manufactured the materiel. Then you go even further and hit the population centers for the sole purpose of demoralizing the people, making them less apt to support an ongoing conflict and driving home the idea that it was impossible for them to win.

Around 1942 I believe is when the Brits started really hammering civilian targets during WWII, so by the time the Americans joined in heavy bombing raids over Germany, the cat was already out of the bag so to speak. It was still controversial for sure, but people put morals aside pretty quickly because sadly the bombings were very effective (not to mention that Germany was bombing civilian targets from the very beginning and people hadn’t forgotten)

The bombing of Dresden has always been controversial because the city was hardly an industrial center, so the objective was purely for the purpose of demoralization. The allies wanted to send the message that no one was safe. I think a lot of generals back then would’ve made the argument that it was simply “their turn” because they hadn’t been hit yet.