The 1925 Geneva Protocol did not focus on World War I's terrible new 20th-century technologies that made 19th-century military tactics obsolete and led to mass slaughter: advancements in barbed wire, machine guns, and artillery led to incomprehensible and horrible effects on combatants. It was the impact of gas use on both the Western and Eastern fronts that led to the prohibition on chemical and biological warfare, even though it had led to only about one percent of the deaths there. The protocol viewed gas warfare as different from the other methods of mass killing, and banned the use of "asphyxiating, poisonous or other gases" as well as "bacteriological methods."
At least three strains of reasoning were advanced by the International Red Cross, religious leaders, the military, and politicians to help mobilize public opinion in favor of a special prohibition against chemical and biological warfare.
First, there were the unique methods of killing—and the special suffering—caused by the gases of World War I, which were first used by the Germans in the battle of Ypres in 1915 and then by all the armies. Chlorine damaged ears and eyes and caused death by asphyxiation. It was subsequently replaced by phosgene, a colorless gas that damaged the lungs and caused suffocation in a delayed reaction after exposure. Mustard gas caused blistering of the outer body and internal organs, especially the lungs. Death might come only after prolonged agony. And those who survived often had serious respiratory and other health issues for the rest of their lives.