How did chemical weapons become weapons of mass destruction during World War I?

Chemical weapons emerged as weapons of mass destruction (WMD) during World War I when they were introduced in their modern form, characterized by the large-scale, systematic deployment of toxic agents against military forces.1

The transition to WMD status was driven by several factors:

• Massive Scale of Use: The German army initiated modern chemical warfare on April 22, 1915, at Ypres, Belgium, by releasing chlorine gas across a 6-km front. By the end of the war, all major powers had employed more than two dozen different chemical agents, including phosgene and mustard gas.21
• High Casualty Rates: The widespread use of these weapons resulted in approximately 1,300,000 casualties and 91,000 fatalities. Mustard gas alone is estimated to have caused up to 90 percent of all chemical casualties during the conflict.13
• Industrialized Warfare: The development of these weapons required the mobilization of national scientific and industrial resources, such as the work of Fritz Haber, to synthesize chemicals on a scale capable of affecting entire battlefields.4

The horrific impact of these weapons on soldiers led to a global moral consensus that their use was inhumane, ultimately resulting in the 1925 Geneva Protocol and later the Chemical Weapons Convention.13