United States of America
Hulton Archive/ Stringer/ Getty
Poison gas was first used successfully by German forces at Ypres, Belgium, in 1915. The idea of using gas as a weapon had developed as a means to try and break the deadlock of trench warfare. Chlorine, bromine and mustard gas were all used. While actual fatalities were limited, the psychological effects on soldiers were significant. The gas mask perfectly illustrates the dehumanisation of the individual in this war.