The impact of armed drones during the decade-plus of this intense global counterterrorism campaign is hard to overestimate: Without operational commanders and visionary leaders, terror groups decay into locally focused threats, or disappear altogether. Targeted strikes against al Qaeda leaders and commanders in the years immediately after 9/11 deprived the group of the time and stability required to plot a major strike. But the London subway attacks in July 2005 illustrated the remaining potency of al Qaeda's core in the tribal areas of Pakistan. The threat was fading steadily. But not fast enough. So-called signature strikes -- in which target selection is based not on identification of an individual but instead on patterns of behavior or unique characteristics that identify a group -- accelerated this decline for simple reasons. Targeting leadership degrades a small percentage of a diffuse terror group, but developing the tactical intelligence required to locate an individual precisely enough to stage a pinpoint strike, in a no-man's land half a world away, is time-consuming and difficult. And it's not a perfect science; the leaders of groups learn over time how to operate more securely. Furthermore, these leaders represent only a fraction of the threat: Osama bin Laden might have been the public face of al Qaeda, but he was supported by a web of document-forgers, bombmakers, couriers, trainers, ideologues, and others. They made up the bulk of al Qaeda and propelled the apparatus that planned the murder of innocents. Bin Laden was the revolutionary leader, but it was the troops who executed his vision. Signature strikes have pulled out these lower-level threads of al Qaeda's apparatus -- and that of its global affiliates -- rapidly enough that the deaths of top leaders are now more than matched by the destruction of the complex support structure below them. Western conceptions of how organizations work, with hierarchal structures driven by top-level managers, do not apply to al Qaeda and its affiliates. These groups are instead conglomerations of militants, operating independently, with rough lines of communication and fuzzy networks that cross continents and groups. They are hard to map cleanly, in other words. Signature strikes take out whole swaths of these network sub-tiers rapidly -- so rapidly that the groups cannot replicate lost players and their hard-won experience. The tempo of the strikes, in other words, adds sand to the gears of terror organizations, destroying their operational capability faster than the groups can recover. There are other rationales for these attacks, though. Part of the reason signature strikes have become so prominent in this global counterterror war is, simply put, geography. Local terrorist groups only become international threats if they have leadership that can execute a broad, globalist vision, and if that leadership has the time and space to plot without daily distractions from armies and security services -- as in safe havens like Yemen, Somalia, the Sahel, and the tribal areas of Pakistan. These are exactly the places where the United States cannot apply conventional force and where local governments lack the capability or will to counter the threat. Exactly the places where drones offer an option to eviscerate a growing terror threat that has a dispersed, diffuse hierarchy. The places where signature strikes have proven effective. With more capable security partners, the brutal destruction from drones above might come from more conventional operations on the ground. But, by definition, safe havens aren't penetrable by capable security services. There is an intangible factor that reinforces the effectiveness of signature strikes: the fear factor, coupled with the suspicions and paranoia that result from organizations searching desperately among their ranks to find out who is providing the Americans information so detailed that we can wreak such havoc over such a long period of time. Time and again, intelligence has clearly told us that the adversary dreads these operations -- lethal strikes that come anytime, anywhere, and that eliminate entire swaths of organizations. And these same organizations then turn around and further degrade their operational capability by engaging in savage hunts for leaks. Despite such success, questions about how we should employ them -- or whether we should use them at all -- are coming to dominate debates about signature strikes. When do they end? And is it appropriate to strike groups of people not because we can identify a dangerous individual terrorist among them, but instead simply because a cluster of people bears clear hallmarks -- the "signature" -- that is associated with a terror group. This emerging debate will be colored, rightly, by the fact that, in just a decade, drone technology has proliferated. The technology and its use has far outpaced the development of policy that balances national security, morality, and the certainty that whatever precedent we set will be used, and abused, by the rogues and despots who no doubt will acquire this capability. Before the pendulum swings too far in the other direction, though, away from the unquestionably aggressive use of drones by two consecutive presidents and toward a model that imposes tight limits, we are going to have to answer a simple question or two: When the president receives information that a new group -- maybe not a terror organization, but an evolving militant group -- is plotting to strike America at home or abroad, what do we do? If we strike too soon, we risk alienating a local population and increasing its motivation to target New York. If we strike too late, a nascent group of violent extremists will become operational, a lesson we learned too well 12 years ago. So take off the table the 20th-century notion that drones will become part of a more conventional military structure; they won't. The question for the 21st century is easy to state but hard to answer: Given the lessons of 9/11 and Iraq, when should a president choose preemption? And where? What are the rules for this new war?