Since 2001, flying robots piloted remotely by the US Central Intelligence Agency have killed hundreds of people across the Middle East and South Asia. Among the most recent casualties was Pakistani Taliban leader Hakimullah Mehsud, who was killed last week along with three others in the town of Miranshah, near Pakistan's border with Afghanistan.
In addition to Mehsud, several other Taliban and al-Qaeda commanders have been reported killed by US drone strikes in recent months, including Wali ur Rehman, reportedly the Pakistani Taliban's second-in-command; al-Qaeda number-two Atiyah Abd Rahman; and Qaeed al-Dahab, an al-Qaeda commander in Yemen.
The New America Foundation, which has extensively studied the US drone war, says 28 "senior al-Qaeda operatives" were killed by drones in Pakistan between 2004 and 2013. The US also flies unarmed drones over the Middle East and elsewhere for surveillance.
Last year, US President Barack Obama reportedly told his aides that he is "really good at killing people" with drones, according to a new book on last year's presidential election.
Of course, anti-US fighters aren't the only people who have been killed by American drones: At least 450 civilians also have died, according to a UN report published in October - news that has justifiably sparked outrage from human rights advocates around the world.
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But from a coldly military point of view, has the US drone strategy been effective at disrupting and degrading anti-US fighter groups like al-Qaeda and its affiliates? In the calculus of national security, does the potential "blowback" from using drones outweigh the benefits claimed by US leaders?
'Recruiting tool for al-Qaeda?'
Killing such leaders does little in the long-run to halt terrorism, say critics of US drone policy. In fact, they argue, these leaders are easily replaced, while the drone attacks counterproductively drive locals, hungry for revenge, into the arms of groups like al-Qaeda, and destabilise the countries where they take place.
"Living Under Drones," a report published in September 2012 by researchers from Stanford and New York University law schools, found that "publicly available evidence that the strikes have made the US safer overall is ambiguous at best." Earlier this year, activist Farea al-Muslimi testified before the US Senate that the strikes are "fuelling anti-Americanism" in his home country of Yemen, going so far as to say that the US "has become al-Qaeda's public relations officer."
Baraa Shiban, an anti-drone activist and member of Yemen's National Dialogue, told Al Jazeera that drone strikes that kill civilians are an "easy tool for militants if they want to recruit after that." These groups can then say, "Your government's not doing anything, the United States hasn't even apologised [and] this is the only way to make revenge," explained Shiban.
Furthermore he argued, "the strikes are not even helping the Yemeni government to gain stability and to have a rule of law on the ground … The United States should realise that it's in its interest to have a stable, secure Yemen."
Would-be terrorists have cited the drone war as a motivation. Faisal Shahzad, who attempted to bomb New York City's Times Square in 2010, said in court that "until the hour the US pulls its forces from Iraq and Afghanistan and stops the drone strikes in Somalia and Yemen and in Pakistan … we will be attacking US, and I plead guilty to that."
There's no question that public opinion of the US is low in countries where the US has operated drones. Just 11 percent of Pakistanis have a favourable view of the US - down from 16 percent in 2009.
Data-driven drone defence
Yet Anoop Sarbahi, a political scientist at Stanford University who specialises in geospatial analysis and counterinsurgencies, isn't convinced that drones are ineffective or counterproductive, as their critics claim them to be.
[Drones] disrupt and degrade terrorist organisations, reducing their ability to conduct attacks.
In fact, a study released this summer (the first version of the study was circulated in January 2011) by Sarbahi and RAND Corporation researcher Patrick Johnston examined statistical correlations between the location of drone strikes and the frequency of terrorist attacks nearby, concluding that drones "disrupt and degrade terrorist organisations, reducing their ability to conduct attacks."
"We didn't find any suggestion that [the US use of drones] is actually leading to an increase in militant activities," he told Al Jazeera.
Sarbahi cast doubt on the sources used by researchers and journalists who have argued that drone strikes are counterproductive because they add to the ranks of hardline groups.
"It could be that we are relying too much on anecdotal evidence of people joining terrorist organisations based on the fact that drones are killing people," he said. "We have this example of the Times Square bomber … you can come across a whole bunch of people who claim that. But there could be a larger proportion of people who are not willing to join" militant groups because of the drones. However, Sarbahi stressed that there is insufficient data to draw conclusions about drones' effect on recruitment levels.
"Going and interviewing people in places like [Pakistan's tribal areas], you sometimes forget that it's not like interviewing them in the US, where you just go and randomly select the person and talk to him," Sarbahi added. "The way society functions there and the dominance of violence may actually inhibit people from giving their honest answers on issues."
Drones begetting drones?
Assuming that drone strikes do indeed reduce the frequency of terrorist attacks nearby, is it possible that they have the effect of causing fighters to find new safe havens in other parts of the country?
"Insurgents may increasingly abandon rural areas like FATA in favor of urban areas of the sort that insurgents have traditionally eschewed," Sarbahi and Johnston wrote in their study, "but that may now offer greater protection from drones and other sophisticated countermeasures."
That's already happened to some extent, said the American Enterprise Institute's Michael Rubin. "Drone strikes have driven many of the militant leaders out of the mountains, and into the dense urban jungles of the southern Punjab and Karachi," he told Al Jazeera. "That legacy of drone warfare will only be felt in the decades to come."
There's also the possibility that heavy use of drones will sap the motivation of US policymakers to use non-military tactics to reduce the threat of terrorism. Using drones to kill anti-US fighters thousands of kilometres away requires obvious technical sophistication - but it may still be quicker and simpler than engaging in years of diplomatic legwork or economic development projects.
And by alienating the public in places like Pakistan and Yemen, the US may not be able to use such non-military tactics, even if it wanted to, because popular pressure could dissuade local politicians from cooperating with diplomatic solutions. The US reliance on drones could then become even more pronounced given the lack of alternatives.
Non-military alternatives to the drone war will become more difficult, say opponents of US strategy, because drone strikes are not only affecting hardline groups - they're also corroding the power of governments in the countries where these groups operate.
In Yemen, Shiban said the government "is losing more control of the situation [in areas where drone strikes have taken place. … The United States should realise that it's in its interest to have a stable, secure Yemen. Definitely drone strikes are not helping us to do that."
And in an essay published earlier this year, Michael Boyle - a former counterterrorism adviser to Barack Obama - wrote that "to drain support from the array of militant movements in Waziristan, FATA [semi-autonomous tribal areas of Pakistan] and elsewhere, the Pakistani government must appear as a credible competitor for the loyalties of the population in tribal regions."
Destabilising a nuclear-armed country where military coups are common, Boyle argued, is a foolhardy approach. US drone strikes in Pakistani territory "serve as powerful signals of these governments' helplessness and subservience to the United States."
As a result, he wrote, "a stable set of partnerships for counterterrorism cooperation" become "difficult, if not impossible".
Follow Sam Bollier on Twitter: @SamBollier