Few innovations in modern warfare have seduced US presidents as thoroughly as the Predator drone, upstaged in more recent years by its bigger and even deadlier cousin, the Reaper. Under George W Bush, the Hellfire missiles slung under its wings delivered ‘sudden justice’ to America’s enemies. Barack Obama embraced the technology even more ardently, carrying out eight times more drone strikes in Pakistan than his predecessor and opening new fronts in Yemen and Somalia.

It is not hard to imagine why temptation proved so difficult to resist. With their sleek fuselages and distinctive downward-V tail fins, drones serve as ‘aerial sniper rifles’, allowing the pinpoint application of deadly force without putting American troops in danger. Unlike conventional aircraft, drones can ‘slow down war’ by loitering above a target for hours, ensuring civilian casualties are kept to a minimum. The unblinking stare from their swivelling, robotic eyes ensures the most dangerous militants remain forever on the run. To their promoters, these bulbous-nosed craft are an exemplar of how technological superiority can keep America safe.

Chris Woods, an investigative journalist, takes a wrecking ball to this narrative with his authoritative and admirably even-handed chronicle of America’s secret drone wars. Woods argues that drone strikes have led to five hundred unacknowledged non-combatant deaths, despite ‘fanciful’ CIA claims to the contrary. No longer reserved for ‘high-value targets’, drones have been used to obliterate hazily defined groups of ‘military-aged males’ mourning at funerals or desperately digging people from the rubble left by previous strikes. Operating far beyond public scrutiny, the CIA has used its covert fleet to wage an open-ended campaign of targeted killings that would have been unthinkable prior to 9/11, when American spy chiefs were wary of embracing assassination on an industrial scale.

None of these controversies is new. The strength of Woods’s book lies in the dexterity with which he weaves familiar debates into a meticulous account of the technology’s early development and subsequent proliferation. The CIA flew its first unmanned reconnaissance missions in the Balkans in the 1990s, using a Gnat drone powered by a modified snowmobile engine. In 2000, an unarmed drone yielded tantalising glimpses of Osama bin Laden in Afghanistan – a catalyst for weaponising the Predator. The United States can now fly at least sixty-three simultaneous combat drone patrols twenty-four hours a day in the skies of half a dozen countries.

While much of the controversy around drones centres on their use as assassination tools, Woods points out that four out of five strikes occur on conventional battlefields, where the craft have increasingly assumed the close air-support role traditionally played by helicopters or jets. From the point of view of civilians, drones certainly represent an improvement on the indiscriminate bombing of earlier eras. At the same time, the technology has contributed to an irrevocable blurring of the traditional definition of a conflict zone, with profound implications for the ways wars might be waged in the future.

Although Woods explores all the major theatres in which drones have been deployed, the risks and rewards are starkest in Pakistan. US officials have long maintained that the aircraft rendered the core of al-Qaeda largely impotent before bin Laden was himself killed by US Navy SEALs in 2011. But Woods shows how the programme became increasingly tied to the prosecution of the war in Afghanistan, with the Obama administration turning to drones to kill hundreds of low-level militants suspected of launching cross-border attacks from Pakistani havens. Some missions went horribly wrong – such as on 17 March 2011, when a drone strike on a meeting of suspected insurgents killed dozens of tribal elders. Although some local leaders welcomed the drones’ impact on their enemies in the Pakistani Taliban, the campaign soon became a lightning rod for broader anti-American sentiment. Richard Armitage, former US deputy secretary of state, summed up widespread concern in the State Department over the growing reliance on drones: ‘It was so antiseptic and was so lacking in any oversight, of checks and balances … I thought it could be very easy for the administration to get the US into big trouble.’

Yemen highlighted further dilemmas. In 2011, Obama authorised the killing of US citizen Anwar al-Awlaki, a cleric who had led lunchtime prayers at the Pentagon after 9/11, but who was later alleged to be a senior al-Qaeda operative. Woods has demonstrated that the US launched its global counterterrorism operations with total disregard of international law. The killing of al-Awlaki and other Americans suggested its own constitution had also become a casualty.

Woods has gained impressive access to the facilities where men and women pilot the craft, select their targets and observe in intimate detail the carnage they cause half a world away. Their voices are a reminder that drones have not only eroded the boundaries of battlefields overseas, but also brought war into American homes. ‘I know the feeling you experience when you see someone die,’ wrote analyst Heather Linebaugh. ‘When you are exposed to it over and over again, it becomes like a small video, embedded in your head, forever on repeat.’

At its heart, this book poses fundamental questions about the relationship between technology and conflict. Do drones save lives by providing a new way to kill terrorists who would otherwise have harmed the innocent? Or do they encourage the pursuit of fleeting tactical gains while fuelling the kind of instability that raises the risk of strategic defeat? More problematic still, might a growing global fleet of drones enable countries to wage wars they might otherwise have avoided? Woods leaves the reader in little doubt as to the answers.