THE use of Unmanned Aerial Systems, as the armed forces prefer to call them, is growing. Drones have become today's weapon of choice in counter-terrorism. And over the next 40 years or so, they are expected largely to replace piloted aircraft. In nine years the Pentagon has increased its drone fleet 13-fold and the generals are spending at least $5 billion a year adding to it. The frequency of drone strikes on al-Qaeda and other terrorists that lurk in Pakistan's Federally Administered Tribal Areas (FATA) has risen under Barack Obama to one every four days, compared with one every 40 during George Bush's presidency. In Libya NATO commanders turned to drones when their fast jets failed to find and hit Muammar Qaddafi's mobile rocket launchers. Not everyone feels comfortable with all this. Critics say that the legal and ethical issues surrounding the use of drones have been neglected. Some of those concerns may be exaggerated, but others need to be taken seriously, particularly if, as seems certain, armies will increasingly fight with machines, not men. There are good reasons for using more drones. Cruise missiles and jet fighters work against fixed targets, concentrations of forces or heavy weapons on open ground. They are not as useful, however, in today's “wars among the people” fought against insurgents and terrorists. Drones such as the Predator and the Reaper can loiter, maintaining what one former CIA director described as an “unblinking stare” over a chosen area for up to 18 hours. Thanks to the drone's ability to watch and wait, its “pilot”, often thousands of miles away, can patiently choose the best moment to fire its missiles, both increasing the chances of success and minimising the harm to civilians. That makes the drone the ideal weapon for tracking down and killing terrorists, particularly in places like the FATA where other options, such as sending in special forces, are not politically feasible. Claims in Pakistan that American drone attacks have killed thousands of civilians are undermined by research (see article) carried out at the New America Foundation, a think-tank, suggesting that in the seven years since 2004, 80% of the fatalities have been militants and that last year (thanks in part to intelligence provided by the Pakistanis themselves) fully 95% of them were. The increasing accuracy of these attacks and the evidence that they have helped to weaken al-Qaeda encourage some to believe (not least in the White House) that counter-terrorist campaigns in the future can be waged without the sacrifice of blood and treasure that goes with putting thousands of boots on the ground.

Before that happens, America must square up to some of those ticklish legal and moral questions that drones raise. The United States is surely right to seek to minimise its own casualties, but if war can be waged by one side without any risk to the life and limb of its combatants, has a vital form of restraint been removed? Is the drone “pilot” who clocks off after a day's work a legitimate target for those he has been hunting down? If the drones of the future have the intelligence to act autonomously, who is responsible if a vital algorithm fails to distinguish between a tank and a school bus? Drones throw up a tangle of ethical questions. Only open debate will provide the answers; they cannot be assumed by button-pushers.

Yet the more fundamental argument that armed drones somehow breach the laws of war does not, at present, stand up. There are still plenty of human beings in the operational loop—it takes a team of about 180 to run and service a Predator—and it is clear that the responsibility for the decision to fire a missile rests as much with the pilot in a distant command centre as with a pilot in any cockpit. The legal defence for that missile killing people who have not been proven to be terrorists or who have not been allowed the chance to give themselves up is the same too. America must show that the attack is within its right to self-defence and that it is proportionate.

To improve accountability, control of armed drones flying over Pakistan and Yemen should be transferred from the CIA to the armed forces (which operate them in Afghanistan, Iraq and Libya). The CIA can use drones to spy, but when it comes to warfare, it is less accountable than the military chain of command, less used to applying the rules of war and less inclined to pay compensation to the families of innocent civilians who have been killed. The operation of America's new killing machines must be brought clearly within the law.