Blister agent
Lewisite was first created in 1904.  The United States experimented with using lewisite as a chemical weapon in the 1920s and during World War II.  However, lewisite did not work well because it smelled like geraniums (a type of flower) and made people's eyes water.  Enemy soldiers would smell the chemical and realize their eyes were watering, and would put on gas masks to protect themselves.