During World War II, a top commander in what was then the U.S. Army Air Forces, General Henry "Hap" Arnold, developed a new way to attack U-boat stations and other heavily fortified German positions: he turned old B-17 and B-24 bombers into remotely piloted aircraft and loaded them with explosives. "If you can get mechanical machines to do this," Arnold wrote in a memo to his staff, "you are saving lives at the outset." The missions had a poor track record, but that did not deter Arnold from declaring in 1945 that "the next war may be fought by airplanes with no men in them at all."
Nearly seven decades later, Arnold's prophecy is slowly being realized: armed drones are starting to rule the skies. So far, the United States has had a relative monopoly over the use of such drones, but it cannot count on maintaining that for much longer. Other states are quickly catching up. And although these new weapons will not transform the international system as fundamentally as did the proliferation of nuclear weapons and ballistic missiles, they could still be used in ways that are highly destabilizing and deadly.
Countries will not be deterred from launching drone attacks simply because an adversary has drones in its arsenal, too. If anything, the inherent advantages of drones -- most of all, not placing pilots or ground forces at risk of being killed or captured -- have lowered the threshold for the use of force. Spurred by the United States' example, other countries are likely to threaten or conduct drone strikes in ways that are harmful to U.S. interests, whether by provoking regional adversaries or targeting domestic enemies.