It prohibits the use of "asphyxiating, poisonous or other gases, and of all analogous liquids, materials or devices" and "bacteriological methods of warfare".

[citation needed] This resulted in the development of a range of horrific chemicals affecting lungs, skin, or eyes.

[8] 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.

[6] 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,[11][12] 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.

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 Soviet 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.

[15] 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.

[17][18][19] In 1966, United Nations General Assembly resolution 2162B called for, without any dissent, all states to strictly observe the protocol.

[20] 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.

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.

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.