Diagram of a Great War-era phosgene artillery shell used by the German army.

Phosgene is a simple organic compound that exists as a colorless gas with a smell likened to cut grass. While it is most commonly used as an industrial reagent in the production of certain plastics, it is most widely recognized as a chemical weapon that was employed in high quantities in the Great War. While it is nowhere near as deadly as the sinister VX Nerve Agent, phosgene is cheap and relatively easy to acquire. Phosgene does not share the extreme lethality of its modern counterpart and many of those exposed to the gas have lived to tell the tale, but it is still highly dangerous; up to 170,000 soldiers and French civilians were killed by phosgene in the Great War, and more still were killed in China by Unit 731.

The Spanish in particular, who never felt the sting of the Great War, have few qualms with using phosgene as a weapon and openly admit to fielding phosgene weapons. Spanish military doctrine holds that phosgene is most effective as a means of making large portions of the battlefield "unsuitable for enemy occupation or transit". Documents published by the Spanish Field Artillery Corps contend that phosgene shelling can be an effective means of forcing an entrenched enemy from fortifications or dense vegetation.