More than a hundred years ago, French troops fired the first tear gas grenades at the German enemy. Designed to force people out from cover, tear gas causes tearing and gagging, burning the eyes and skin. Its use has ended in miscarriages, permanent injuries, and death. While all but a few countries have agreed that it is illegal to manufacture, stockpile, or use chemical weapons of war, tear gas continues to proliferate in civilian settings. Today, it is a best-selling form of “less lethal” police force. From Ferguson to the Occupied Territories of Palestine, images of protesters assaulted with “made in the USA” tear gas canisters have been seen around the world. The United States is the largest manufacturer, and Brazil and South Korea are rapidly growing markets, while Britain has found an international audience for its riot control expertise. An engrossing century-spanning global narrative, is the first history of this poorly understood weapon. Anna Feigenbaum travels from military labs and chemical weapons expos to union assemblies and protest camps, drawing on declassified reports and eyewitness testimonies to show how policing with poison came to be.