Chemical warfare (CW) involves using the toxic properties of chemical substances as weapons. This type of warfare is distinct from nuclear warfare, biological warfare and radiological warfare, which together make up CBRN, the military acronym for chemical, biological, radiological, and nuclear (warfare or weapons), all of which are considered "weapons of mass destruction" (WMDs), a term that contrasts with conventional weapons.
The use of chemical weapons in international armed conflicts is prohibited under international humanitarian law by the 1925 Geneva Protocol and the Hague Conventions of 1899 and 1907. The 1993 Chemical Weapons Convention prohibits signatories from acquiring, stockpiling and using chemical weapons in all circumstances except for very limited purposes (research, medical, pharmaceutical or protective).
Definition Chemical warfare is different from the use of conventional weapons or nuclear weapons because the destructive effects of chemical weapons are not primarily due to any explosive force. The offensive use of living organisms (such as anthrax) is considered biological warfare rather than chemical warfare; however, the use of nonliving toxic products produced by living organisms (e.g. toxins such as botulinum toxin, ricin, and saxitoxin) is considered chemical warfare under the provisions of the Chemical Weapons Convention (CWC). Under this convention, any toxic chemical, regardless of its origin, is considered a chemical weapon unless it is used for purposes that are not prohibited (an important legal definition known as the General Purpose Criterion).
About 70 different chemicals have been used or were stockpiled as chemical warfare agents during the 20th century. The entire class, known as Lethal Unitary Chemical Agents and Munitions, has been scheduled for elimination by the CWC.Disarmament lessons from the Chemical Weapons Convention
Under the convention, chemicals that are toxic enough to be used as chemical weapons, or that may be used to manufacture such chemicals, are divided into three groups according to their purpose and treatment: Schedule 1 – Have few, if any, legitimate uses. These may only be produced or used for research, medical, pharmaceutical or protective purposes (i.e. testing of chemical weapons sensors and protective clothing). Examples include nerve agents, ricin, lewisite and mustard gas. Any production over must be reported to the Organisation for the Prohibition of Chemical Weapons (OPCW) and a country can have a stockpile of no more than one tonne of these chemicals. Schedule 2 – Have no large-scale industrial uses, but may have legitimate small-scale uses. Examples include dimethyl methylphosphonate, a precursor to sarin also used as a flame retardant, and thiodiglycol, a precursor chemical used in the manufacture of mustard gas but also widely used as a solvent in inks. Schedule 3 – Have legitimate large-scale industrial uses. Examples include phosgene and chloropicrin. Both have been used as chemical weapons but phosgene is an important precursor in the manufacture of plastics, and chloropicrin is used as a fumigant. The OPCW must be notified of, and may inspect, any plant producing more than 30 tons per year.
Chemical weapons are divided into three categories: The Chemical Weapons Convention (CWC) at a Glance Category 1 – based on Schedule 1 substances Category 2 – based on non-Schedule 1 substances Category 3 – devices and equipment designed to use chemical weapons, without the substances themselves
History
Simple chemical weapons were used sporadically throughout antiquity and into the Industrial Age.Samir S. Patel, "Early Chemical Warfare – Dura-Europos, Syria," Archaeology, Vol. 63, No. 1, January/February 2010, https://archive.archaeology.org/1001/topten/syria.html (accessed October 3, 2014) It was not until the 19th century that the modern conception of chemical warfare emerged, as various scientists and nations proposed the use of asphyxiating or poisonous gasses.
Multiple international treaties were passed banning chemical weapons based upon the alarm of nations and scientists. This however did not prevent the extensive use of chemical weapons in World War I. Chlorine gas, among other chemicals, was used by both the Allied and Central powers to try to break the stalemate of trench warfare. In most cases the gasses used did not kill, but instead incapacitated or caused permanent or temporary injuries. Historians have reached a wide range of estimates on gas casualties, ranging from 500k to 1.3 million casualties directly caused by chemical warfare agents during the course of the war, of which 2-4% died. A minimum of around 1300 civilians were injured due to the use of the weapons, and at least around 4000 were injured during weapon production. Gas casualties were a small fraction of the overall human impact of the war, but the horrifying effects of such weapons gave them a strong psychological impact.
The interwar years saw the occasional use of chemical weapons during the Second Italo-Ethiopian War by the Italians, while the Japanese made use of chloroacetophenone to suppress a native rebellion in Formosa in 1930, and later made use of mustard gas, lewisite, and irritant agents during the Second Sino-Japanese War. The Chinese, often lacking protective gear such as gas masks, were usually forced to retreat against CW attacks.
World War II Although significant effort went into the development and stockpiling of chemical weapons in World War II, they saw no battlefield use in the European Theatre. Nazi Germany dedicated much research to the development of nerve agents,Corum, James S., The Roots of Blitzkrieg, University Press of Kansas, 1992, pp. 106–107. but never used them even during the final defense of the fatherland. It has been suggested that Adolf Hitler's own experience with poison gas during WWI and fears that the Allies would retaliate with their own chemical weapons were the main reasons chemical weapons were never used by Nazi Germany. According to military historian Ian V. Hogg, the Germans were reluctant to use nerve agents since they couldn't find any defense or antidote for them. They also feared that the ban on scientific journals about organophosphates in Britain meant that the Allies also discovered nerve agents. In reality, they were unaware of the G-series until an ammunition dump captured in April 1945 was examined, while the ban on scientific publications was meant to prevent Axis nations from learning about advances on the development of insecticides and herbicides to boost food production; nonetheless, the Allies made comprehensive plans for defensive and retaliatory use of chemical weapons, and stockpiled large quantities."Paxman and Harris", Pakistan pp. 132–135.Callum Borchers, Sean Spicer takes his questionable claims to a new level in Hitler-Assad comparison, The Washington Post (April 11, 2017).
The Imperial Japanese Army, as part of the Axis, used chemical weapons in China and Burma until 1943. After President Roosevelt threatened the Japanese with retaliation in kind in 1942, they were forced to limit their usage in the Pacific Theatre. Chemical weapons were frequently used against both the Kuomintang and People's Liberation Army troops in China, while Unit 731 carried experiments with CW agents on prisoners of war in 1940 and 1943. By 1944, the Japanese gradually ceased production of chemical munitions in favor of conventional high-explosive and incendiary shells, while keeping a large enough stockpile for retaliatory use.
However, Nazi Germany extensively used poison gas against civilians, particularly Jews, in the Holocaust. Vast quantities of Zyklon B and carbon monoxide gas were used in the systematic extermination of some three million victims. This remains the deadliest use of poison gas in history.Patrick Coffey, American Arsenal: A Century of Weapon Technology and Strategy (Oxford University Press, 2014), pp. 152–154.James J. Wirtz, "Weapons of Mass Destruction" in Contemporary Security Studies (4th ed.), ed. Alan Collins, Contemporary Security Studies (Oxford University Press, 2016), p. 302.
Post-War French forces frequently used chemical weapons, such as CN2D and sarin, against Algerian rebels during the Algerian War of Independence. The attacks killed and wounded thousands of Algerian civilians in 5,000 to 10,000 separate operations.
Small amounts of chemical agents were employed by Egyptian forces during the North Yemen civil war between 1963 and 1967, resulting hundreds of casualties. In one of the largest chemical attacks, the United States dropped over 90,000 tons of chemical agents, mostly Rainbow Herbicides but tear gas as well during the Vietnam War.
During the Iran–Iraq War, some 100,000 Iranian troops were casualties of Iraqi chemical weapons. Iraq also used mustard gas and nerve agents against the Kurdish population killing more than 5,000 people and injuring many in the 1988 Halabja chemical attack.On this day: 1988: Thousands die in Halabja gas attack, BBC News (March 16, 1988).
The Cuban intervention in Angola saw limited use of organophosphates.
On 22 September 1991, during the battle of Šibenik, Federal Yugoslav authorities accused Croatian forces of employing non-lethal chemical agents to storm the naval base in the city's harbour.
Terrorist groups have also used chemical weapons, notably in Japan in the 1990s, illustrated by the 1994 Matsumoto incident and the 1995 Tokyo subway sarin attack.Seto, Yasuo. "The Sarin Gas Attack in Japan and the Related Forensic Investigation." The Sarin Gas Attack in Japan and the Related Forensic Investigation. Organisation for the Prohibition of Chemical Weapons, June 1, 2001. Web. February 24, 2017.
21st century Syria
The Ba'athist regime in Syria used sarin, chlorine, and mustard gas in numerous deadly chemical attacks against civilian populations during the Syrian civil war.Syria Used Chlorine in Bombs Against Civilians, Report Says, The New York Times, Rick Gladstone, August 24, 2016 retrieved August 25, 2016.
Russia During the Russian invasion of Ukraine, Russia has been reported to deploy CS gas through K-51 grenades dropped by unmanned drones. On 13 December 2024, the Ukrainian military stated that over 4,800 incidents involving chemical weapons against Ukrainian forces have been recorded since the war began, which resulted in over 2,000 Ukrainian soldiers having been hospitalized, and 3 deaths. The use of gas was often hidden by heavy Russian "intense artillery, rocket, and bomb attacks”, forcing Ukrainian soldiers out of their positions. They saw less use of chemical gas in cold weather, as it reduced the effectiveness of the K-51 gas grenades. A 2024 US aid package to Ukraine included "nuclear, chemical and radiological protective equipment".
Sudan Amnesty International published a report in September 2016 titled Scorched Earth, Poisoned Air, which documented at least 30 suspected chemical weapons attacks by the Sudanese Armed forces (SAF)in the Jebel Marra region of Darfur between January and September 2016. According to the report, an estimated 200–250 people, many of them children, were killed as a result of exposure to chemical agents.
Following the outbreak of armed conflict between the SAF and the Rapid Support Forces (RSF) in April 2023, further allegations emerged concerning the use of industrial chemicals as weapons. In May 2025, the United States Department of State determined that the SAF had used chemical weapons during 2024. This determination resulted in the imposition of sanctions under the Chemical and Biological Weapons Control and Warfare Elimination Act, including measures targeting SAF leadership such as Abdel Fattah al-Burhan.