Autonomous weapons are robotic systems that, once activated, can select and engage targets without further intervention by a human operator. Advances in computer technology, artificial intelligence, and robotics may lead to a vast expansion in the development and use of such weapons in the near future. Public opinion runs strongly against killer robots. But many of the same claims that propelled the Cold War are being recycled to justify the pursuit of a nascent robotic arms race. Autonomous weapons could be militarily potent and therefore pose a great threat. For this reason, substantial pressure from civil society will be needed before major powers will seriously consider their prohibition. However, demands for human control and responsibility and the protection of human dignity and sovereignty fit naturally into the traditional law of war and imply strict limits on autonomy in weapon systems. Opponents of autonomous weapons should point out the terrible threat they pose to global peace and security, as well as their offensiveness to principles of humanity and to public conscience.