1899 - Hague Convention banned " the use of projectiles the sole object of which is the diffusion of asphyxiating or deleterious gasses."
1912 - French police used grenades filled with ethyl bromoacetate, or tear gas, to capture a gang of Parisian bank robbers. The French army may have used similar grenades in August 1914 against the German army advancing from Belgium.
1914 Oct. 27 - The Germans fired 3000 105-mm artillery shells filled with dianisidine chlorosulphonate, a form of tear gas, at British troops near Neuve-Chapelle. The Berlin chemistry professor Walter Nernst had recommended this byproduct of the German dye industry as a possible weapon. The Kaiser Wilhelm Institute for Physical Chemistry and Electrochemistry in Berlin continued research on chemical weapons. Professor von Tappan developed a special 150mm artillery shell, named the T-shell in his honor, that held 7 pounds of xylyl bromide and an explosive charge for a splinter effect. A lead lining prevented contact between the TNT and the chemicals. The explosive charge technically made the shell exempt from the Hague Convention.
1915 Jan. 31 - The Germans fired 18,000 T-shells at Russian positions at Bolimov on the Eastern Front, but the cold temperatures prevented the vaporization of the xylyl bromide, and the Russians repelled the attack.
1915 Apr. 22 - The Germans released 168 tons of chlorine gas at Langemarck near Ypres. This type of gas was developed by Dr. Fritz Haber, a German army reservist and assistant to vonTappan in Berlin. He proposed gas cylinders as a delivery system, exempt from Hague Convention, and became head of the Chemical Warfare Department in the Prussian Ministry of War. He arranged for Pioneer Regiment 35 to move 1600 large and 4130 small cylinders to the Ypres front in March, and waited for one month until the winds were satisfactory.
1915 May - The Allies lost 5900 casualties due to 3 gas attacks at Ypres, but quickly began to wear primitive gas masks as a defense.
1915 Sep. 25 - British at Loos released 150 tons of chlorine gas from 5500 cylinders. The chlorine was an asphyxiant that damaged lungs and eyes
1916 Feb. 22 - French fired the first phosgene gas shells at Verdun, timed to burst overhead. The French would manufacture 4 million of such Vincennite shells during the war, but they casued few casualties
1916 June 22 - Germans fired 110,000 shells of diphosgene "Green Cross" asphyxiant at French forces near Verdun, along with tear gas shells
1917 July - Germans fired the first mustard gas shells at Ypres. The mustard gas was a colorless and oderless blistering agent that affected exposed skin, and remained deadly for days in the bottom of trenches and shell holes. It was the most deadly of the poison gases used in the war.
1917 Sept. - Germans used mustard gas against Russians at Riga.
1918 May 24 - British launched the first gas beam attack, by massing cylinders on naroow gauge tram cars just behind the forward trenches.