Why and how chemical weapons were banned

Chemical weapons were banned in two main steps. The 1925 Geneva Protocol banned using them in war, and the 1993 Chemical Weapons Convention (in force since 1997) banned owning them at all. The trigger was revulsion at World War I. The ban lasted mostly because major powers decided these weapons weren't worth much to them.

How

• Early rules: The 1899 Hague Declaration banned shells filled with asphyxiating gas, and the 1907 Hague Convention banned “poison or poisoned weapons.” In 1915 at Ypres, Germany got around the shell ban by releasing chlorine from cylinders. By 1918, gas had caused roughly 1.3 million casualties and about 90,000 deaths.
• 1925 Geneva Protocol: It banned use only. Countries could still develop, make and stockpile chemical weapons, and many reserved the right to retaliate in kind. That made it in effect a no-first-use pledge. It failed to stop Italy in Ethiopia, Japan in China, Egypt in Yemen, or Iraq against Iran and the Kurds (Halabja, 1988, about 5,000 dead).
• Chemical Weapons Convention: Halabja and the end of the Cold War led to this treaty. It bans development, production, stockpiling, transfer and use. It requires states to destroy their stockpiles, and the OPCW checks compliance with inspections, including short-notice challenge inspections. 193 states are parties; the holdouts are Israel (signed but not ratified), Egypt, North Korea and South Sudan. The last declared stockpile, held by the US, was destroyed in July 2023.

Why it worked when most weapons bans haven't

1. Poor military value: Wind, gas masks and protective gear made gas less effective than high explosives against prepared troops. After 1945, nuclear weapons did the deterrence job, so giving up gas cost the great powers little.
2. Fear of retaliation: In WWII, both sides held stockpiles and neither used gas on the battlefield.
3. Fear of a “poor man's atomic bomb”: Strong states wanted to deny weak states a cheap weapon that could offset their military advantage.
4. An old taboo against poison, made stronger by the horror of the trenches.

My interpretation (not settled fact) is that the moral taboo explains the public pressure, but reasons 1 and 2 explain why governments actually signed. The ban still isn't airtight: Syria used chemical weapons after joining in 2013, and Russia used Novichok nerve agents against Skripal and Navalny.