Geneva Protocol
Negotiation history: In the Hague Conventions of 1899 and 1907, the use of dangerous chemical agents was outlawed. In spite of this, the First World War saw large-scale chemical warfare. France used tear gas in 1914, but the first large-scale successful deployment of chemical weapons was by the German Empire in Ypres, Belgium in 1915, when chlorine gas was released as part of a German attack at the Battle of Gravenstafel. Following this, a chemical arms race began, with the United Kingdom, Russia, Austria-Hungary, the United States, and Italy joining France and Germany in the use of chemical weapons.
This resulted in the development of a range of horrific chemicals affecting lungs, skin, or eyes. Some were intended to be lethal on the battlefield, like hydrogen cyanide, and efficient methods of deploying agents were invented. At least 124,000 tons were produced during the war. In 1918, about one grenade out of three was filled with dangerous chemical agents. Around 1.3 million casualties of the conflict were attributed to the use of gas, and the psychological effect on troops may have had a much greater effect.
As protective equipment developed, the technology to destroy such equipment became a part of the arms race. The use of deadly poison gas was not only limited to combatants in the front but also civilians, as nearby civilian towns were at risk from winds blowing the poison gases through. Civilians living in towns rarely had any warning systems about the dangers of poison gas, as well as not having access to effective gas masks. The use of chemical weapons employed by both sides had inflicted an estimated 100,000-260,000 civilian casualties during the conflict. Tens of thousands or more, along with military personnel, died from scarring of the lungs, skin damage, and cerebral damage in the years after the conflict ended. In 1920 alone, over 40,000 civilians and 20,000 military personnel died from the chemical weapons effects.
The Treaty of Versailles included some provisions that banned Germany from either manufacturing or importing chemical weapons. Similar treaties banned the First Austrian Republic, the Kingdom of Bulgaria, and the Kingdom of Hungary from chemical weapons, all belonging to the losing side, the Central powers. Russian bolsheviks and Britain continued the use of chemical weapons in the Russian Civil War and possibly in the Middle East in 1920.
Three years after World War I, the Allies wanted to reaffirm the Treaty of Versailles, and in 1922 the United States introduced the Treaty relating to the Use of Submarines and Noxious Gases in Warfare at the Washington Naval Conference. Four of the war victors, the United States, the United Kingdom, the Kingdom of Italy and the Empire of Japan, gave consent for ratification, but it failed to enter into force as the French Third Republic objected to the submarine provisions of the treaty.
At the 1925 Geneva Conference for the Supervision of the International Traffic in Arms the French suggested a protocol for non-use of poisonous gases. The Second Polish Republic suggested the addition of bacteriological weapons. It was signed on 17 June.

Historical assessment: Eric Croddy, assessing the Protocol in 2005, took the view that the historic record showed it had been largely ineffectual. Specifically it does not prohibit:

use against not-ratifying parties
retaliation using such weapons, so effectively making it a no-first-use agreement
use within a state's own borders in a civil conflict
research and development of such weapons, or stockpiling them
In light of these shortcomings, Jack Beard notes that "the Protocol (...) resulted in a legal framework that allowed states to conduct [biological weapons] research, develop new biological weapons, and ultimately engage in [biological weapons] arms races".
As such, the use of chemical weapons inside the nation's own territory against its citizens or subjects employed by Spain in the Rif War until 1927, Japan against Seediq indigenous rebels in Taiwan (then part of the Japanese colonial empire) in 1930 during the Musha Incident, Iraq against ethnic Kurdish civilians in the 1988 attack on Halabja during the Iran–Iraq War, and Syria or Syrian opposition forces during the Syrian civil war, nor use on Black Lives Matter protestors in the United States did not breach the Geneva Protocol.
Despite the U.S. having been a proponent of the protocol, the U.S. military and American Chemical Society lobbied against it, causing the U.S. Senate not to ratify the protocol until 1975, the same year when the United States ratified the Biological Weapons Convention.

Violations: Several state parties have deployed chemical weapons for combat in spite of the treaty. Italy used mustard gas against the Ethiopian Empire in the Second Italo-Ethiopian War. In World War II, Germany employed chemical weapons in combat on several occasions along the Black Sea, notably in Sevastopol, where they used toxic smoke to force Russian resistance fighters out of caverns below the city. They also used asphyxiating gas in the catacombs of Odesa in November 1941, following their capture of the city, and in late May 1942 during the Battle of the Kerch Peninsula in eastern Crimea, perpetrated by the Wehrmacht's Chemical Forces and organized by a special detail of SS troops with the help of a field engineer battalion. After the battle in mid-May 1942, the Germans gassed and killed almost 3,000 of the besieged and non-evacuated Red Army soldiers and Soviet civilians hiding in a series of caves and tunnels in the nearby Adzhimushkay quarry.
During the 1980-1988 Iran-Iraq War, Iraq is known to have employed a variety of chemical weapons against Iranian forces. Some 100,000 Iranian troops were casualties of Iraqi chemical weapons during the war.

Subsequent interpretation of the protocol: In 1966, United Nations General Assembly resolution 2162B called for, without any dissent, all states to strictly observe the protocol. In 1969, United Nations General Assembly resolution 2603 (XXIV) declared that the prohibition on use of chemical and biological weapons in international armed conflicts, as embodied in the protocol (though restated in a more general form), were generally recognized rules of international law. Following this, there was discussion of whether the main elements of the protocol now form part of customary international law, and now this is widely accepted to be the case.
There have been differing interpretations over whether the protocol covers the use of harassing agents, such as adamsite and tear gas, and defoliants and herbicides, such as Agent Orange, in warfare. The 1977 Environmental Modification Convention prohibits the military use of environmental modification techniques having widespread, long-lasting or severe effects. Many states do not regard this as a complete ban on the use of herbicides in warfare, but it does require case-by-case consideration. The 1993 Chemical Weapons Convention effectively banned riot control agents from being used as a method of warfare, though still permitting it for riot control.
In recent times, the protocol had been interpreted to cover non-international armed conflicts as well international ones. In 1995, an appellate chamber in the International Criminal Tribunal for the former Yugoslavia stated that "there had undisputedly emerged a general consensus in the international community on the principle that the use of chemical weapons is also prohibited in internal armed conflicts." In 2005, the International Committee of the Red Cross concluded that customary international law includes a ban on the use of chemical weapons in internal as well as international conflicts.
However, such views drew general criticism from legal authors. They noted that much of the chemical arms control agreements stems from the context of international conflicts. Furthermore, the application of customary international law to banning chemical warfare in non-international conflicts fails to meet two requirements: state practice and opinio juris.  Jillian Blake & Aqsa Mahmud cited the periodic use of chemical weapons in non-international conflicts since the end of WWI (as stated above) as well as the lack of existing international humanitarian law (such as the Geneva Conventions) and national legislation and manuals prohibiting using them in such conflicts. Anne Lorenzat stated the 2005 ICRC study was rooted in "'political and operational issues rather than legal ones".

State parties: To become party to the Protocol, states must deposit an instrument with the government of France (the depositary power). Thirty-eight states originally signed the Protocol. France was the first signatory to ratify the Protocol on 10 May 1926. El Salvador, the final signatory to ratify the Protocol, did so on 26 February 2008. As of April 2021, 146 states have ratified, acceded to, or succeeded to the Protocol, most recently Colombia on 24 November 2015.

Reservations: A number of countries submitted reservations when becoming parties to the Geneva Protocol, declaring that they only regarded the non-use obligations as applying with respect to other parties to the Protocol and/or that these obligations would cease to apply with respect to any state, or its allies, which used the prohibited weapons. Several Arab states also declared that their ratification did not constitute recognition of, or diplomatic relations with, Israel, or that the provision of the Protocol were not binding with respect to Israel.
Generally, reservations not only modify treaty provisions for the reserving party, but also symmetrically modify the provisions for previously ratifying parties in dealing with the reserving party.