Was mustard gas used in WWI?

Mustard gas was used during World War I; the Germans were the first to use the gas as a chemical warfare weapon in 1917. Mustard gas was a useful method for breaking the deadlock resulting from trench warfare, where soldiers avoided "no man's land," the area between two trenches.

Mustard gas and other chemical agents were used with devastating effects during World War I, resulting in casualties numbering around 100,000 and around a million injuries to civilians and soldiers. Mustard gas harmed the enemy both during and after the initial attack, because the gas lingered in the ground for weeks.