Then WWII came along. Waging war under the Geneva Protocol prevented all sides from from using chemical weapons, but it didn't keep them from developing far more indiscriminate and powerful weapons.
It did, for the most part. A blast radius is a blast radius. Whether I have a grenade or a nuclear bomb, I can be pretty sure what's going to happen when I use it. Sure, I can flatten a city block with an artillery strike, or I can the surgical precision that we have been capable of for some time now thanks to increasingly sophisticated guiding systems. The exceptions to this are things like Agent Orange, which continues to cause health problems through things like birth defects (so do the chemical weapons from WWI, actually). Aside from nuclear weapons, there isn't much out there that can cause serious health problems to people who weren't even alive during war.
While death by gas was certainly a gruesome way to die, I don't believe it was any more so than to die watching one's intestines spill out of an open belly, or feeling the searing heat of a flame thrower on your face.
Sure, all three are unpleasant. So is bleeding out and all kinds of trauma possible with everything from bullets to debris. The main difference is that it is quite easy to die almost instantaneously from just about all weapons used in war. And as medical care has increased in sophistication (even since Mogadishu, which was the impetus for cutting-edge clotting technology for first aid in urban warfare), those who don't die are able to receive morphine and medical treatment to be stabilized and live. Does this mean war isn't gruesome and horrifying? Of course not. But gas changes the level drastically.
The gruesomeness of battle casualties hadn't changed, just the method and nature.
Which changes the gruesomeness of battle casualties. Trauma is trauma. I can get a scratch or have my leg shot off by a .50 cal. round, but the damage to tissue is similar in both cases (especially from a medical viewpoint). Gas is poison. It works via a fundamentally different process than other weapons. It doesn't cause damage to tissue the way bullets, shrapnel, bombs, even flame do. All these destroy tissue through trauma. The same antibiotics that can prevent infection from an open wound can do so for severe burns. The same clotting agents that can stop massive blood loss from a bullet wound can do so for shrapnel. Gases don't work like this. If they do external damage, is secondary. You die without even the natural painkillers provided by shock and adrenaline, in minutes or more, with nothing anybody can do. If you don't die, there are a myriad of health problems you are likely to face.
Gassing isn't really a great method. From a psychological warfare point of view, it's likely to cause damage to both sides. It's much harder to control, dangerous to transport, and has no advantages over bombs/missiles. The "shock and awe" that we want isn't massive destruction but taking out specific targets. It's easy to level cities to the ground.
it poses no greater threat to civilians than does conventional warfare
It does. If we want to kill civilians, then sure- we don't need gas. But if we don't want to, it's much harder not to with gas. And nothing, other than nuclear bombs, (which aren't conventional) causes the long-lasting health problems that can physiologically affect those who haven't even been born. Conventional warfare involves trauma, from cuts and scrapes to severed limbs and gaping wounds. These can be horrific, bad but manageable, lethal, relatively harmless, etc. Gases are toxins. If they don't kill you, the damage they do is to internal organs. They can cause cancer, damage to organs that isn't detected until later, birth defects, a wide range of respiratory problems, neurological problems, and so much more. You can't get much more gruesome and, as there's no advantage to using gas, there's no reason to cause such horror.
I doubt many people writing up the Geneva Protocol could have envisioned the massive losses of life that were to take place in the Tokyo fire bombings
The battles of Verdun and the Somme were possibly the most bloody, brutal, and devastating in human history. More people were killed in a day than in the bombing of Dresden. More people were killed in Verdun than in both the Dresden bombings and the Tokyo firebombing put together. The shelling of the opposite side during WWI followed by the "over the top" orders where men charged machine guns and were mowed down because they used tactics that hadn't been viable for at least 40 years and were less than ideal since the late 18th century. There's a reason it was called The War to End All Wars. And the use of gas was a big one.
