Scharre, a former U.S. Army Ranger, has thought more than most about the implications of autonomous weapons. He has spent time not only among their designers and operators but also with those so alarmed by the prospect of machines making life-and-death decisions that they wish to ban them—or, at the very least, impose strict limits on their use. In this comprehensive analysis, Scharre moves beyond the clichés of “killer robots” to explore the complexity of human-machine interactions, distinguishing systems that still partly rely on humans from those that are completely autonomous and highlighting the importance not only of machines’ capabilities but also of human fallibilities. Scharre recognizes the advantages of remote-control systems and doubts whether it would be possible to craft and enforce a ban. But he also warns against being “seduced by the allure of machines—their speed, their seeming perfection, their cold precision.” New technologies are most likely to reduce the harm caused by war if they incorporate the principles of proportionality and discrimination, which are already central to the established laws of armed conflict.