As the public increasingly questioned the war in Vietnam, a group of American scientists deeply concerned about the use of Agent Orange and other herbicides started a movement to ban what they called “ecocide.” David Zierler traces this movement, starting in the 1940s, when weed killer was developed in agricultural circles and theories of counterinsurgency were studied by the military. These two trajectories converged in 1961 with Operation Ranch Hand, the joint U.S.-South Vietnamese mission to use herbicidal warfare as a means to defoliate large areas of enemy territory. Driven by the idea that humans were altering the world’s ecology for the worse, a group of scientists relentlessly challenged Pentagon assurances of safety, citing possible long-term environmental and health effects. It wasn’t until 1970 that the scientists gained access to sprayed zones confirming that a major ecological disaster had occurred. Their findings convinced the U.S. government to renounce first use of herbicides in future wars and, Zierler argues, fundamentally reoriented thinking about warfare and environmental security in the next forty years. Incorporating in-depth interviews, unique archival collections, and recently declassified national security documents, Zierler examines the movement to ban ecocide as it played out amid the rise of a global environmental consciousness and growing disillusionment with the containment policies of the cold war era.