Answer :
Final answer:
Poison gas was used by the Germans in World War I but later declared illegal by the Geneva Convention.
Explanation:
The weapon that was used by the Germans in World War I but later declared to be illegal by the Geneva Convention was poison gas. Poison gas, including chlorine gas, mustard gas, and phosgene, was employed as an instrument of war by both sides. It caused severe injuries, including burns, blindness, and respiratory problems, and was responsible for a significant number of casualties.
