This essay was adapted from Drones and Terrorism, the new book by Arc editor-at-large Nicholas Grossman, published in March 2018 by I.B.Tauris, (available now).

Upon taking office, Donald Trump changed a lot, repudiating much of Obama’s legacy. But not the drone campaign.

Under both presidents, the CIA and U.S military fire missiles from unmanned aircraft at suspected terrorists and insurgents, primarily in Pakistan and Yemen, as well as a few in Somalia. These attacks take place outside active military theaters, such as Afghanistan, which makes them extrajudicial, outside the laws of war.

In Obama’s eight years, American drones launched 5.6 attacks per month in Pakistan and Yemen. The Trump administration has maintained a similar rate, 4.6 per month. Trump has focused more on Yemen and less on Pakistan, though that continues a trend that began in Obama’s second term.

Because the drone campaign falls in a gray area of international law — not explicitly illegal, but not legal either — and because attacks sometimes kill civilians, the policy is widely criticized, often on moral grounds. However, most of these criticisms judge drone strikes in a vacuum (are drone strikes good or bad?) rather than weighing them against alternative options.

When the United States identifies an individual as an active member of a terrorist organization, or discovers someone is plotting to execute a terrorist attack against an American or allied target, there are at least five potential responses:

Drone strike

Airstrike from a manned aircraft

Ground raid to capture or kill the suspect

Encourage local forces to handle the situation

Leave the suspect alone and focus on anti-terrorism

Each comes with trade-offs.

Drone Strikes v. Attacks from Manned Aircraft

American drone strikes sometimes kill civilians. In one of the most egregious incidents, the United States killed 12 people on their way to a wedding in Yemen in December 2013. But this is a problem for any weapon fired from distance, not drones per se.

Manned and unmanned aircraft use similar missiles and targeting equipment, and are therefore equally likely to attack the wrong target. The United States does not deliberately target civilians as part of asymmetric warfare against terrorist and insurgent groups — there is no strategic value in doing so — but the U.S. is willing to accept some civilian deaths as collateral damage. In the few instances in which American aircraft fired upon purely civilian targets, the reason was faulty intelligence, and both manned and unmanned planes are equally subject to this problem.

Robotic airplanes do not need to eat, sleep, or use the bathroom. Ground-based drone operators can attend to bodily needs, or change shifts due to fatigue. This means UAVs can remain in the air longer than any manned aircraft, with pilots constantly operating at peak capacity, which allows them to be more selective about the timing of attacks and reduces the likelihood of error.

Due to limited flight time and fear of enemy fire, the pilot of a manned plane is more likely to attack a target when the opportunity arises. Unmanned planes, by contrast, can remain in the air for 36 hours or more, allowing them to wait for greater certainty about a target’s identity, and for targeted individuals to be isolated from non-combatants. As a result, drone strikes cause fewer civilian deaths than attacks from manned aircraft.

As the following table shows, an increasing reliance on drone strikes coincided with a decrease in civilian casualties in Afghanistan. According to statistics from United States Air Force Central Command, weapons fired from unmanned aircraft increased by 42% from 2011 to 2012, going from 5.45% of total airstrikes to 12.37%. Meanwhile, civilian casualties (including both deaths and injuries) declined by 42%, while civilian deaths from airstrikes declined by 46%. Though 2012 featured fewer total weapons released by aircraft, this cannot explain the decline in civilian casualties, as the rate of both civilian casualties and civilian deaths per airstrike decreased.

These numbers come from Afghanistan, an active military theater, while the drone campaign refers to attacks by unmanned aircraft outside of official warzones. And the rates of civilian deaths from drone strikes in Pakistan (17.11%-19.74%) or Yemen (11.06%-11.14%) measured by independent analysts are considerably higher than the U.S. military’s reported rate in Afghanistan (about 4.22%, based on the above data from 2010–2012).

Perhaps the military provides a more generous accounting of its own activities relative to the independent assessment of scholars and journalists. Or it could be because American intelligence is more accurate and extensive in locations with a large overt ground presence relative to areas of covert activity. Regardless, while U.S. government claims regarding civilian casualties from extrajudicial drone strikes deserve skepticism — since the United States has a clear incentive to downplay accidental damage to civilians — there is no clear incentive for internal stats to portray one type of aircraft as more likely to harm civilians than another.

Comparing one year of military statistics to another shows a decline in the rates of both civilian deaths and casualties when drone usage spiked, suggesting UAVs are better at minimizing civilian casualties than manned aircraft.

An A-10 Thunderbolt (manned aircraft) fires a missile. Photo: Master Sgt. Robert Wieland U.S. Air Force

Drone Strikes v. Ground Raids

While drone strikes are often cheaper than ground raids, they eliminate the possibility of capturing suspects. “Snatch-and-grab” missions risk soldiers’ lives, and captured terrorists need to be imprisoned and perhaps put on trial. Both cost time, effort, and money, and risk giving a terrorist a public platform.

Additionally, imprisoning terrorist suspects may come with political costs. The United States has faced considerable criticism for holding suspects without trial at Guantanamo Bay and secret CIA “black sites.”

However, killing, rather than attempting to capture suspects, forfeits potential intelligence gains. Destroying targets from the air eliminates the chance to interrogate them for information or find material revealing details about the group’s membership, finances, strategy, and intended targets. Documents and hard drives captured in the U.S. raid on Osama bin Laden’s compound in May 2011 provided a “treasure trove” of information that helped the United States understand and further weaken al Qaeda.

On October 5, 2013, American forces captured Abu Anas al Libi in Libya. Al Libi was a senior al Qaeda operative wanted by the United States for his role in the 1998 embassy bombings in Kenya and Tanzania. He pleaded not guilty to terrorism charges in a federal court in New York City on October 15, 2013, and died in a New York hospital from complications related to liver cancer in January 2015, shortly before the start of his trial.

Prosecuting rather than killing al Libi avoided criticism associated with extrajudicial force, and, given his affiliation with al Qaeda dating back to the 1990s, interrogating him likely yielded valuable information.

However, in the same month the United States captured al Libi, American forces failed to capture a leader of al Shabaab in Somalia. U.S. Navy SEALs faced unexpected resistance from al Shabaab fighters, and pulled back to reduce the risk of American deaths and widespread civilian casualties.

These two October 2013 ground raids demonstrate the trade-off between undertaking a risky capture mission, which can produce intelligence and lead to legal prosecution, and executing a drone strike, which is more likely to neutralize the target, but eliminates the possibility of intelligence gains.

Ground raids also offer the possibility of greater precision. Soldiers can identify their target in person, and avoid shooting civilians in the target’s proximity. The bin Laden raid illustrates the potential advantages of ground raids relative to airstrikes. Fewer civilians died than almost certainly would have if the U.S. bombed bin Laden’s compound, and the Special Operations unit known as SEAL Team 6 was able to acquire valuable intelligence.

However, there is a risk ground operations could fail disastrously. If the bin Laden raid represents a cleanly executed ground raid that highlights the potential benefits, the October 1993 attempt to capture senior members of the Somali National Alliance in Mogadishu illustrates the downsides of failure.

In what is popularly known as the Black Hawk Down incident, American Special Operations forces ended up in a prolonged firefight with Somali militants and an unruly mob. As many as 500 Somalis died and many more were injured, most of whom were civilians. The incident ended with 18 dead American troops dragged through the streets of Mogadishu, images of which appeared on television around the world.

A raid on an al Qaeda base in Yemen on January 28, 2017 — the first military action Trump authorized as president — highlights the trade-offs. One U.S. Navy officer died, and three SEALs were injured. American forces killed at least 15 civilians, including an 8-year-old girl. However, they also killed three senior al Qaeda operatives, and seized computers and cell phones.

Compared to drone strikes, ground raids’ potential upside and downside are both greater.

A Black Hawk helicopter during a training exercise. Photo: Staff Sgt. Roy Lynch, U.S. Air Force

Working with Local Governments

Instead of launching airstrikes or deploying Special Operations forces, the United States could work through local governments, asking them to kill or capture suspected militants. Governments exercising authority over their own territory would create less anger against the United States, and potentially less of a backlash overall.

However, the United States already works with the Pakistani government, worked with the government of Yemen until it collapsed in January 2015, and Somalia is a failed state lacking a central government capable of securing its territory.

In both Pakistan and Yemen, the groups America targets with drone strikes are enemies of the local government as well. Al Qaeda, the Pakistani Taliban, the Haqqani network, and other groups based in the Federally Administered Tribal Areas (FATA) near the Afghan border oppose the government of Pakistan. The Pakistani military has launched numerous operations in the FATA, and, from 2003–2015, groups based there killed over 6,000 Pakistani soldiers and 20,000 Pakistani civilians.

Similarly, al Qaeda in the Arabian Peninsula (AQAP) opposed the government of Yemeni President Ali Abdullah Saleh before he was ousted by 2011 Arab Spring protests, and opposed Saleh’s successor, Abdu Rabbu Mansour Hadi, before he resigned under pressure from Houthi rebels in 2015. Under both Saleh and Hadi, Yemeni forces launched attacks against AQAP, and the government arrested and jailed al Qaeda operatives.

Pakistan, and especially Yemen, have less powerful militaries than the United States, which means they cannot attack with the same precision or without accepting greater risk to their soldiers, and both have supported American attacks on their territory.

In 2010, a diplomatic cable released by WikiLeaks showed President Saleh giving the United States an “open door” for drone strikes in Yemen, and agreeing to take responsibility for the attacks to avoid the potential political problems that could arise from Yemeni citizens finding out foreign forces were launching airstrikes on their territory. The Hadi government continued cooperating with the United States, but since Hadi’s resignation, it is no longer clear America has the permission of the local governing authority to conduct drone strikes, and the United States withdrew from bases in Yemen in March 2015.

The ongoing civil conflict, and lack of a central government, means Yemen more closely resembles a failed state like Somalia, where there is no authority capable of killing or capturing most suspected terrorists. Though the United States pulled back, American drones have continued launching strikes over Yemen, including a missile that killed AQAP leader Nasir al Wuhayshi in June 2015.

Secret CIA documents and Pakistani diplomatic memos obtained by the Washington Post in 2013 revealed the Pakistani government, like the government of Yemen, endorsed American drone strikes on its territory. Additionally, Pakistani officials received CIA briefings before the U.S. launched attacks. Some of the earliest Predator flights over Afghanistan and Pakistan took off from Pakistani airstrips, though the United States moved these launches to Afghanistan in an attempt to reduce the appearance of Pakistani complicitness.

Pakistani officials publicly oppose drone strikes, which aligns with Pakistani public opinion. However, Pakistan continued accepting billions in U.S. aid (until Trump froze it in January 2018), the two countries’ intelligence agencies continue sharing information, and Pakistan has the ability to shoot down foreign drones on its territory but chooses not to.

At the very least, Pakistan tacitly supports the American drone campaign, despite public statements otherwise. Nevertheless, in 2013 the United States agreed to limit attacks on Pakistani soil to high value targets, in response to criticism from Pakistani military officials that frequent attacks risked creating more enemies than they killed.

When the local government is both willing and able to arrest or kill terrorist suspects on its own, the United States usually prefers not to intervene. For example, in April 2015, Saudi Arabia announced it had arrested 93 people with ties to ISIS and disrupted a plot to attack the American embassy in Riyadh with a suicide car bomb.

However, countries have accused U.S. counterterrorists of arresting suspects on their territory without permission. In 2009, an Italian court tried in absentia and convicted 22 CIA operatives and a U.S. Air Force colonel on kidnapping charges for the 2003 capture of Hassan Mustafa Osama Nasr on Italian soil.

Nevertheless, there is no publicly known instance of the United States firing missiles at terrorist suspects in countries where the local government was willing and able to capture or kill the target themselves and denied the United States permission to operate on their soil.

U.S. Army Col. Robert Waltemeyer (left), meets with Pakistan Army Brig. Gen. Asad Shahzada. Photo Credit: SGT. Richard Andrade, 16th Mobile Public Affairs Detachment

Focus on Anti-terrorism

It is widely assumed the United States will attack suspected terrorists in some manner, and drone strikes are often an attractive option due to the costs and benefits relative to manned airstrikes, ground raids, and working through local governments. However, the U.S. could choose to focus exclusively on disrupting active plots and strengthening homeland defenses, rather than also killing suspects in an attempt to degrade enemy networks. Refraining from extrajudicial killings would reduce a source of political opposition to the United States.

After the September 11th attacks, the United States improved anti-terrorism defenses. The newly created Department of Homeland Security (DHS), Transportation Security Administration (TSA), and National Counterterrorism Center (NCTC) worked to strengthen security at airports and other ports of entry, as well as various symbolic and soft targets.

These defenses are hardly foolproof — for example, in 2015 tests, undercover DHS investigators were able to sneak mock explosives or banned weapons through TSA check points 67 out of 70 times — but their combined efforts, along with new protections set up by local police departments and private security firms, have hardened many potential targets.

To protect against the threat of terrorism, the U.S. increased intelligence capacity and improved intelligence sharing, both among American agencies and with foreign partners. Post-9/11 reforms aimed to break down information silos and increase cooperation between the FBI and CIA, and created over 100 Joint Terrorism Task Forces to coordinate activity among federal, state and local intelligence and law enforcement agencies. The United States also increased intelligence sharing with international partners such as the U.K., Germany, Israel and Saudi Arabia. In 2015, America spent approximately twice as much on intelligence as it did in 2001.

In the aftermath of 9/11, the government expanded intelligence agencies’ powers. In particular, the NSA increased collection of telephone metadata (phone number called and the length of the call, but not the content) and monitoring of internet activity. In a program known as PRISM that began in 2007, the NSA — along with its British equivalent, GCHQ — gathers information on internet users with assistance from nine major technology companies, including Google, Facebook, Microsoft, Yahoo and Apple.

Information from these secret programs, as revealed by former NSA contractor Edward Snowden, contributes significantly to intelligence reports. Data collected by PRISM appeared in the President’s Daily Brief 1,477 times in 2012, and provided the lead item in 1 out of 7 NSA intelligence reports. American officials testified in 2013 that the surveillance programs foiled over 50 terrorist plots in the United States and abroad, including a plan to bomb the New York Stock Exchange.

This claim cannot be independently verified, because the details of these alleged plots remain classified, and it is impossible to know whether the attacks would have succeeded if the NSA did not collect information on telephone and internet communications. Additionally, as critics argue, whatever security these surveillance programs provide may not be worth the sacrifice of privacy they require.

Nevertheless, when taking into account all post-9/11 reforms and expanded powers, it is safe to say the American intelligence community improved its ability to anticipate and disrupt terrorist attacks before they can succeed.

September 11th also changed the American public’s mindset, making the country less vulnerable. Before 9/11, most people assumed airplane hijackers wanted to commandeer a flight to Cuba, use the passengers as hostages in exchange for ransom money, or gain attention and leverage hostages to advance political demands, such as the release of prisoners. 9/11 taught the world that hijackers might kill everyone on board, and others as well.

This experience increased citizen vigilance. For example, passengers disabled both “shoe bomber” Richard Reid in December 2001, and “underwear bomber” Umar Farouk Abdulmutallab in December 2009, as they tried to ignite explosives.

With hardened targets, improved intelligence, and a more vigilant populace, the United States has become less vulnerable to terrorism, which arguably reduces the need to kill or capture members of terrorist organizations abroad.

There is little doubt drone strikes create blowback. Mounting evidence indicates considerable anger in Pakistan and Yemen over these attacks, and both al Qaeda and ISIS have used the American drone campaign as a recruiting tool (though it’s impossible to know how many people this convinces who would not be convinced otherwise). To undermine this recruiting pitch and reduce blowback, the United States could refrain from firing at terrorist suspects abroad, trusting anti-terrorism defenses to disrupt active plots.

Some terrorists will try to attack the United States and its allies in the future because an American drone strike killed someone they love, or because they are angry about the United States violating their country’s sovereignty or killing civilians. However, their anger is almost certainly a reaction to the U.S. using force and killing people, rather than a reaction specific to drones.

Any forceful action the United States takes would anger some people, especially those directly harmed by it, and the level of anger is probably a factor of whom the U.S. kills rather than the weapon used to kill them. If the United States targeted terrorist suspects with missiles fired from manned aircraft instead of drones, or killed a similar number of people in Pakistan and Yemen with ground raids, anti-American groups would design recruitment pitches accordingly.

One of al Qaeda’s central grievances against the United States is American support for Saudi Arabia, Egypt, and other governments in Muslim-majority countries, which indicates working with local governments creates blowback as well. Most people opposed to the drone campaign are opposed to the United States killing people extrajudicially, and would not become more pro-American if the U.S. switched to an alternative method of killing suspected terrorists and insurgents.

But what if the U.S. stopped entirely? Committed members of al Qaeda or ISIS wouldn’t quit, but the groups might have a harder time recruiting.

However, if the United States identifies a suspected terrorist and does nothing, there’s a risk the suspect executes a successful attack in the future. The probability that happens is nearly impossible to determine, but it’s safe to say the officials who declined to capture or kill the terrorist when they had the chance would face immense criticism.

Every option comes with trade-offs. The most appropriate action in any given situation depends on specific circumstances, as well as which risks you’re willing to tolerate.

Here’s a possible middle ground: Limit drone strikes to confirmed high value targets, and take extra care to minimize civilian casualties. That could reduce blowback and lessen the appeal of al Qaeda and ISIS’ recruitment efforts, thereby weakening anti-American groups and making it easier for foreign governments and populations to cooperate with the United States.

But in some situations, considering the upsides and downsides of alternatives, a drone strike is probably the best — or least bad — option.