In September 1910, the activist Roger Casement arrived in the Amazon jungle on a mission for the British government: to investigate reports of widespread human-rights abuses in the forests along the Putumayo River. Casement was outraged by what he uncovered: nearly thirty thousand Indians had died to produce four thousand tons of rubber for Peruvian and British commercial interests, under the brutal rubber baron Julio Cesar Arana. In 1912, Casement's seven-hundred-page report of the Putumayo violence set off reverberations throughout the world. Drawing on a wealth of original research,...