Fully autonomous weapons are one of the most alarming military technologies under development today. As such there is an urgent need for states, experts, and the general public to examine these weapons closely under the Martens Clause, the unique provision of international humanitarian law that establishes a baseline of protection for civilians and combatants when no specific treaty law on a topic exists. This report shows how fully autonomous weapons, which would be able to select and engage targets without meaningful human control, would contravene both prongs of the Martens Clause: the principles of humanity and the dictates of public conscience. To comply with the Martens Clause, states should adopt a preemptive ban on the weapons’ development, production, and use.