Chemical warfare during the First World War involved the deliberate use of toxic chemical substances to injure, incapacitate or kill enemy personnel. Chemical agents were generally delivered through artillery shells, gas cylinders, mortar projectiles and other weapons. The best known agents included chlorine, phosgene and mustard gas. Poison gas was not necessarily intended to kill every person exposed; many agents were designed or used to cause respiratory injury, blindness, skin damage, incapacitation or long term medical problems. Chemical warfare became one of the most distinctive technologies of the conflict because it introduced a new category of battlefield hazard, although chemical weapons had existed in experimental or limited forms before 1914. Their use led to rapid development of protective equipment and countermeasures.
The first major large scale use of poison gas in the First World War occurred at the Second Battle of Ypres in April 1915, when German forces released chlorine gas against Allied positions. Earlier chemical agents and experiments had occurred, but the Ypres attack demonstrated the potential of industrially produced toxic gases as battlefield weapons. Chlorine is a highly reactive gas that can damage the respiratory system.
When released under suitable wind conditions, a gas cloud could drift towards enemy positions. The initial Allied troops affected at Ypres lacked adequate protection and suffered severe casualties, although the exact mortality from the attack was lower than the numbers of people affected. After this event, all major armies increasingly developed their own chemical weapons and defensive systems.
Chlorine was followed by other agents that were more difficult to detect or more persistent. Phosgene was a particularly dangerous choking agent that could cause severe lung damage, sometimes with symptoms appearing after a delay. Mustard gas, first used extensively by German forces in 1917, was a blister agent that could burn the skin, damage eyes and injure the respiratory tract.
Mustard gas could contaminate terrain and equipment for extended periods, making it particularly useful for denying areas rather than simply producing immediate casualties. Chemical agents differed greatly in persistence, toxicity, delivery method and medical effects. Their military usefulness therefore depended on weather, terrain, protective equipment and the tactical objective of the attack.
Gas attacks created unusual battlefield conditions because weather could influence whether an agent reached its intended target. Early chlorine attacks released gas from cylinders, requiring favourable winds and exposing the attacking side to the risk that changing winds could blow the cloud back towards its own troops. Artillery shells containing chemical agents later provided greater tactical flexibility because they could deliver gas without relying entirely on a favourable wind. Gas could be used against trench lines, artillery positions, communication routes and rear areas. Chemical bombardments could force soldiers to wear respirators for prolonged periods, reducing comfort, visibility and sometimes combat effectiveness even when the agent did not cause severe poisoning.
Protective equipment evolved rapidly. Soldiers initially used improvised methods such as cloths soaked in substances intended to neutralise chlorine, but armies soon introduced purpose designed gas masks and respirators. These devices filtered contaminated air and became increasingly sophisticated during the war. Gas alarm systems were also developed, including bells, rattles and shouted warnings. Soldiers were trained to put on masks quickly and maintain them during attacks. Protective clothing became important against persistent agents such as mustard gas. Medical units developed procedures for treating chemical injuries, although treatment options were limited. The arms race between chemical weapons and protective technology meant that gas warfare became a continuous cycle of attack, detection, protection and tactical adaptation.
Chemical warfare produced severe physical effects. Depending on the agent and dose, victims could suffer coughing, choking, breathing difficulties, vomiting, eye damage, skin blistering, lung injury and temporary or permanent disability. Some agents caused delayed symptoms, making diagnosis difficult. Soldiers who survived exposure could require prolonged medical care. Gas casualties were not necessarily fatal, and many victims survived with injuries. The psychological effects were also significant because gas could be difficult to see or smell and could penetrate defensive positions. Soldiers therefore had to respond immediately to warnings even when no obvious enemy weapon was visible. This created a persistent sense of vulnerability in trench environments.
Chemical weapons were used by both sides and became increasingly integrated into artillery operations. By the later stages of the war, gas shells were often used alongside conventional high explosive shells. Chemical warfare was not the single dominant cause of death on the Western Front, because artillery and other conventional weapons caused far greater overall casualties. Nevertheless, gas was militarily significant because it could suppress troops, contaminate positions, disrupt operations and impose burdens on the enemy. Chemical warfare also had an enduring cultural and political impact. Images of blinded soldiers and accounts of gas attacks contributed to the perception of the First World War as a conflict in which industrial science had been applied to mass destruction.
The First World War stimulated international opposition to chemical warfare and influenced later arms control. The Hague Conventions had already restricted certain methods of warfare before 1914, and the use of poison gas generated controversy because of existing legal and ethical debates. After the war, the 1925 Geneva Protocol prohibited the use of chemical and biological weapons in war for states that became parties to it, although it did not comprehensively prohibit their development or stockpiling. Chemical weapons remained part of military arsenals in subsequent decades. The First World War therefore demonstrated both the destructive potential of chemical technology and the difficulty of controlling weapons once states considered them strategically useful.
