I’m very curious about this. It was considered acceptable to burn 10,000s of civilizations to death as collateral damage during bombing raids but the US never attempted to “smoke out” combatants on the pacific islands. Why? Was it actually considered immoral? Did we simply not want to break treaties? Was it a game of “if we do it, they will too?” Or was it simply not done because it would have been ineffective and not worth the cost?
u/Georgy_K_Zhukov discusses the use of chemical weapons after the 1925 Geneva Protocol, as well as the US' own plans to possibly deploy it in the Pacific theater if WW2 had continued.
One thing I notice from the answer is that chemical weapons are dangerous to the user, not just while deploying but also storage and transportation. Bombers could probably have used it in the bombing campaigns on the Japanese mainland but then it would probably be less effective than just bombing.
This is a little delayed, but I just want to note that the US did not just immediately begin firebombing and nuclear bombing. In the early days of the war, before it had entered into it, FDR issued statements condemning city bombing, and it was only through a number of gradual steps that the US began to embrace indiscriminate bombing (steps that were largely first taken by the British and Germans).
The moves towards firebombing and atomic bombing were not uncontroversial; the Secretary of War himself tried to put a halt to the firebombing, unsuccessfully (it was an operational choice made by the military, not the President or his cabinet). The atomic bombing was seen as an extraordinary move by most of those who were involved in the planning to use it — an unusual tactic meant to produce an unusual response — and there were concerns that if not handled poorly, it would be seen as morally problematic. And, indeed, there were a significant fraction of Americans (some 20% or so) who thought, even in the wake of victory, that it was not a moral use of force.
Which is just to say, I think we are at times too quick to imagine that the firebombings were seen as easily acceptable, and that the atomic bombings were seen similarly. We have been taught not to judge the morality of the past too harshly, which is perhaps a good historical instinct (understanding is more important than judgment, I agree, but I don't think that means we can never "judge"), but a side-effect of that effort is to overestimate how homogenous the people of the past were on controversial issues like this (that is, we end up constructing these people as having no moral qualms with firebombing or atomic bombing in our attempt to avoid judging them for doing both). In reality it took quite a lot of time before these kinds of tactics were seen as things states might do, and a lot of rhetorical and strategic "work" to justify them (e.g., by making claims that it wasn't really cities that were being targeted, but "workers' housing" and "cottage industries" and "strategic assets" and so on), and it was all still controversial, even when these things were (rightly or wrongly) widely considered crucial to victory.
Nicholas Baker's Human Smoke is a problematic and justly controversial work (it lets the Germans off way too easily in some ways), but it tries in its own literary-historical way to show how the British in particular gradually (but deliberately) made their way towards these tactics. Sven Lindqvist's A History of Bombing does similar work, and goes through the Cold War. Both do a good job of indicating what kinds of strategic thinking, historical circumstances, and rhetorical work had to be done to make these tactics seem "acceptable."
The other answer gives links to a good thread about the US back-and-forth in thinking about chemical warfare. As it notes, it was hardly absolute — Marshall definitely contemplated using gas warfare to "smoke out" the Japanese during the invasion and island hopping — and one can easily imagine a world in which the US ended up deciding to use chemical weapons as well, and doing the necessary "work" to justify it.