After hearing about Dresden/Tokyo, etc. on Reddit for so long, I decided to form my own opinion and do my own research. I was familiar with the Blitz, and the concept of total war (i.e. "gloves are off on all sides" warfare), but I was unaware of the Coventry Blitz. Similarly, I did not know about the Rotterdam Blitz, when, on May 14, 1940, Nazi warplanes razed Rotterdam to the ground while the city was surrendering.
With atrocities like the Blitzes and Pearl Harbor fresh in the Allies' minds, I think it would have been logical if a small number of people had seen the firebombings in places like Dresden, Kobe, Hamburg and Tokyo as excessive and disproportionate. Especially in the era of total war.
It is often said on Reddit (and mocked by /r/badhistory), that the firebombing of Dresden on February 13-15, 1945 was a war crime since the Allies deliberately targeted civilians for no reason, and the city served no military purpose whatsoever. I was skeptical, so I did some digging. Sure enough, true to the principles of total war, Dresden was a key manufacturing, logistics and railway hub for the Eastern Front. It just also happened to be a city with an immense amount of arts and culture. However, to say that Dresden was purely a civilian city doesn't jive with how the Nazis pursued total war.
I like to watch old clips of news broadcasts from Operation Desert Shield/Storm on Youtube. It was clear from hearing Gen. Schwartzkopf, Gen. Kelly, BGen. Neal, etc. speak that the commanders took pains to avoid indiscriminate bombing or targeting civilians, to avoid the kind of bad publicity that B-52 saturation bombing in Vietnam generated. Such pains were magnified by the bombing of the baby milk factory and the Amiriyah shelter bombing.
U.S. commanders were keen to point out that Hussein weaved his war machine within the civilian infrastructure, such as the communications bunker in the basement of the al-Rashid Hotel. It turned out that the Finnish manufacturer of the Amiriyah shelter admitted (in an interview with a newspaper in Finland) that the Iraqi Army used the bottom floor of the shelter as a command post, and secretly placed civilians in the top floor at night, unbeknownst to the Coalition.
And yet, the Coalition backed off striking places like the baby formula factory or the Amiriyah shelter more often than not, to avoid bad press in the west as well as in the Arab World. If Desert Storm had been fought by WWII total war standards, Baghdad would have been razed to the ground and hundreds of thousands of civilians would have died.
Perhaps after WWII and the advent of nuclear weapons, it was finally decided that human beings are worth more alive than dead (as globalization spread). Or, maybe nuclear weapons and submarine-launched ICBMs became far, far more efficient than total war could ever be. In any case, I don't think there have been any large-scale wars that utilize the principle of total war, since WWII. Maybe the Korean War.
At any rate, the reason why I went on this tangent is because we are of a generation that has not seen the horrors of total war like our grandfathers and great-grandfathers. We weren't there, we don't know what it was like to survive the Blitz in Tube stations, or see half your city wiped off the face of the earth by B-29 firebombing runs. So that tints our perception of history. Maybe Dresden/Hamburg/Tokyo/etc. wasn't seen as excessive or as a war crime back then. I want to know when it started to be considered as such.
One of the more useful resources I encountered on this point was the excellent documentary The Fog of War, centered largely around conversations conducted with Robert McNamara. McNamara is most famous for his role in the escalation of the Vietnam War, but in my mind he is a central figure in considering the morality of saturation bombing by the United States.
McNamara worked under the somewhat infamous General Curtis LeMay. LeMay was the father of the doctrine of saturation bombing, and McNamara worked under his command conducting statistical analyses to study this strategy. McNamara would later go on to denounce this doctrine in stark terms. From the Fog of War documentary:
LeMay said, "If we'd lost the war, we'd all have been prosecuted as war criminals." And I think he's right. He, and I'd say I, were behaving as war criminals. LeMay recognized that what he was doing would be thought immoral if his side had lost. But what makes it immoral if you lose and not immoral if you win?
Of course, this is McNamara many, many years after the firebombing of Japan, with McNamara in his twilight years, likely trying to salvage his legacy by demonstrating remorse, though that's just speculation on my part. The fact that at the time he supported the logistics of firebombing suggests he did not have such a serious moral qualm at the time, however.
We can find a more useful measure of the time line of when the American Department of Defense began to critically evaluate whether firebombing, or just general "saturation bombing" constituted a war crime.
This evaluation may have began with the United State Strategic Bombing Survey, initiated by the US Secretary of War Stimson at the direction of President Roosevelt. This study concluded that saturation bombing failed to achieve many of its strategic goals, mostly relating to the productivity of Axis powers' war economies. If we interpret the above quote from General LeMay literally, we can see that this report in many ways began to implicitly call into question the morality of saturation bombing by calling into question its strategic soundness. This study was released in 1944. Interestingly, no such study was conducted by DoD to evaluate the effectiveness of the extensive aerial bombing campaign of North Vietnam, though by then the US military largely used the parlance of "strategic bombing" rather than "saturation bombing" to describe its aerial bombing doctrine.
So the short answer is that it may have been apparent to the US military during World War II, and it at least was implicitly considered distasteful by DOD at the time of Vietnam War.
Sources:
Looking more at the European theatre, the area bombing campaign certainly was seen as disproportionate or immoral by contemporary observers. Things are a little murky though because of the differences between stated aims, actual aims, and actual methods. It's also important to note that the law on protection of civilians was more hazy in WW2 than now, so using the term "war crime" can be anachronistic.
Quotes here are from Bomber Command by Max Hastings.
In the UK, bomber command held to the idea of using bombing to break the enemy's morale, by making conditions unbearable for civilians. This would clearly be considered a war crime by current standards. And at the time it was widely deemed unacceptable, including by Churchill and the US establishment. However early attempts at precision bombing proved futile. The smallest target the bombers could reliably hit was a city. Since the means weren't up to targeting anything other than cities, that's what the bombers targeted.
In theory this was aimed at the destruction of military targets, but this was effectively a smokescreen. In practice, they destroyed cities. Since this accorded with what Bomber Command wanted to do anyway they were happy to continue. When it entered the war, the USAAF was drawn into this approach. While it professed to be hitting military targets, in practice this was indistinguishable from destroying cities. Only later did it develop a more targeted (and, arguably, effective) approach, particularly its campaign against German oil infrastructure. Bomber Command resisted committing resources to this campaign, almost to the point of insubordination.
Questions of proportionality, and even effectiveness, were not high up for Bomber Command, and not much more important for US commanders.
This approach certainly drew criticism in the UK at the time, including in Parliament (I can't find the name of the most notable MP right now) and, oddly, the chaplain of Bomber Command itself. So questions over its morality were being raised, but probably not being given a great deal of attention.
As for Dresden itself, as an operation, the bombing was not notably different from the bombing of dozens of other cities. The aims, methods, and even level of destruction, were similar. However two things made it notable. The first was that it came at a time when Germany was clearly collapsing. The stated objective, which was essentially to cause disruption of German logistics and communications, was harder to justify as proportionate. Second, Dresden was a city known by educated people in England, a familiar name (unlike, say, Darmstadt), one with emotional resonance.
For the first time since the bomber offensive began, on the news of the destruction of Dresden a major wave of anger and dismay swept through Whitehall and the Air Ministry, echoed in Parliament, and finally reached the gates of High Wycombe. Urgent questions were asked by important people about the reasons for destroying the city.
Concern was heightened by the release of an Associated Press dispatch from [Supreme Headquarters Allied Expeditionary Force] on 17 February, in which a correspondent reported that 'Allied air chiefs' had at last embarked on 'deliberate terror bombing of German population centres as a ruthless expedient to hasten doom'.
Churchill composed a (somewhat self-serving) memo for the Chiefs of Staff Committee and the Chiefs of Air Staff:
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 ... The destruction of Dresden remains a serious query against the conduct of Allied bombing ... I feel the need for more precise concentration upon miltary objectives, such as oil and communications behind the immediate battle-zone, rather than on mere acts of terror and wanton destruction, however impressive.
Max Hastings argues that Dresden was a turning point for opinions of the saturation bombing campaign, a point when its realities could no longer be ignored or excused so easily, and when a considerable ambivalence entered the national view of the campaign.