One hundred years ago, French troops fired tear gas grenades into German trenches. Designed to force people out from behind barricades and trenches, tear gas causes burning of the eyes and skin, tearing, and gagging. Chemical weapons are now banned from war zones. But today, tear gas has become the most commonly used form of "less-lethal" police force. In 2011, the year that protests exploded from the Arab Spring to Occupy Wall Street, tear gas sales tripled. Most tear gas is produced in the United States, and many images of protestors in Tahrir Square showed tear gas canisters with "Made in USA" printed on them, while Britain continues to sell tear gas to countries on its own human-rights blacklist.An engrossing century-spanning narrative, Tear Gas is the first history of this weapon, and takes us from military labs and chemical weapons expos to union assemblies and protest camps, drawing on declassified reports and witness testimonies to show how policing with poison came to be.