Tear gas, a noxious agent that causes tearing, vomiting and pain, was first used in combat by the French military during World War One 100 years ago. It was soon co-opted by the U.S. Chemical Warfare Service for use as a crowd control agent.
After being initially introduced as a replacement for poison gas after that substance was banned from battlefields, tear gas was soon being used used to quell large crowds in the 1920's and 1930's that gathered in the midst of food scarcity and economic uncertainty.
Its use continued throughout the 1960s, being used to corral anti-war protestors, most notably at the University of California, Berkeley. Despite now being banned from wartime use, tear gas is still in use for domestic crowd control, most recently seen during recent protests in Ferguson, Missouri.