At Strasbourg, in 1675, a Franco-German accord prohibited the use of poisoned bullets for the duration of the war between the two parties. Article 16 of Lieber's 1863 Code reads, “Military necessity … does not admit of the use of poison in any way.” In 1901, twentythree of twenty-eight states attending the 1899 Hague Peace Conference ratified Declaration (IV, 2) Concerning Asphyxiating Gases. By 1907, four more states had either ratified or signed adhesions to the declaration. (The United States was the sole nation not to sign.) “The contracting Powers,” the 1899 declaration reads, “agree to abstain from the use of projectiles the sole object of which is the diffusion of asphyxiating or deleterious gases.” According to 1907 Hague Regulation IV, Article 23, “It is especially forbidden – (a) To employ poison or poisoned weapons.”
In late 1914, however, amid the futile slaughter of [World War I] trench warfare, the traditional legal and moral restraints on the use of poison gas began to erode under the pressure of military necessity…. The German High Command had interpreted the Hague gas-projectile declaration as banning only the release of lethal gases from shells specifically designed for that purpose…. [The chemist Fritz Haber, winner of the 1918 Nobel Prize in chemistry] proposed instead that chlorine be released directly from pressurized gas cylinders, allowing the wind to carry the poisonous cloud over the enemy's trenches. This tactic offered a number of potential advantages: chlorine released directly from cylinders would blanket a far larger area than could be achieved with projectiles, and the gas would dissipate rapidly, allowing the affected areas to be occupied by friendly troops.
By that point in 1914, both Germany and France had already fired thousands of artillery rounds of tear gas, but lethal gases had not been used. Now, General Erich von Falkenhayn, chief of the German General Staff, selected Ypres, Belgium, for first use of the chlorine gas against the enemy, intending to reduce a nine-mile bulge of Allied trench line into the German lines. Informed of the pending use of poison gas, the local commander, General Bertthold von Deimling, was at first resistant.