At 6:18 a.m. on July 31, a CIA drone fired the two Hellfire missiles that killed al-Qaeda leader Ayman al-Zawahiri, a former deputy to Osama bin Laden. Since 9/11, the United States has conducted over 14,000 drone strikes like this against suspected terrorist targets. Countries such as Iran, Turkey, Nigeria and Egypt­ have also acquired armed drones and conduct their own strikes.

But do armed drone operations reduce terrorism, or do they actually make countries more vulnerable to it?

To find out, we analyzed patterns of terrorism in 18 countries — every country that has fielded armed drones to date. The evidence reveals that obtaining armed drones reduces the amount of terrorism a country experiences. Armed drones may raise ethical concerns but appear to be an effective counterterrorism tool.

What drone pessimists believe

Some analysts argue that the use of drones increases terrorism for two main reasons.

First, drones can cause "blowback" among civilian populations, when drones kill or psychologically terrify noncombatants and violate countries' sovereignty. For example, data collected by the Bureau of Investigative Journalism suggests that US drone strikes have killed up to 2,220 civilians since 2010, including up to 450 children. Blowback from drone strikes could motivate civilians to directly aid terrorist groups by joining them, providing material support or even carrying out lone-wolf attacks.

Second, drones may increase terrorism by empowering lower-level militants with a greater preference for violence. Drone strikes often target terrorist leaders — like Zawahiri — rather than rank-and-file members. This means a successful strike can have an unintended consequence: undermining the control that leaders have over their subordinates. So a drone strike can potentially backfire because lower-level militants are often younger and have a more limited understanding of the strategic pitfalls of attacking civilians, in comparison to the more experienced bosses they replace.

What drone optimists argue

Alternatively, drone optimists argue that drones decrease terrorism for two main reasons.

First, drones can disrupt terrorist groups by increasing the security risk to militants, making it harder for them to operate. For example, fear of drone strikes can cause terrorists to restrict their movements, reduce their communications, close their training camps, and become more distrustful of allies and potential recruits.

Second, drone strikes may degrade terrorist groups by physically removing leaders and key operators from the battlefield. Highly skilled individuals that can build bombs, fly planes, speak multiple languages, forge documents and avoid detection are critical to carrying out terrorist attacks.

What does the research say?

Most of the evidence on drone effectiveness, to date, comes from scholarship about the US drone program. However, armed drones have proliferated rapidly around the world over the past decade. It's now possible to analyze the counterterrorism impact of armed drones beyond just the US context.

To do so, we've studied the full universe of cases: all 18 countries that acquired armed drones between 2001 and 2019, and the 11 countries that conducted drone strikes against any target during this period. We also examined the longer-term (yearly) impact of drones on terrorism. Some drone pessimists believe that the negative effects of drones only materialize in the long-term. But previous research focuses on the short-term (monthly, weekly or even daily) impact of drones on terrorism, which leaves analysis on the longer-term strategic impacts of drones unclear.

We surveyed open-source materials to identify which countries fielded armed drones and when they obtained this capability. Using the Global Terrorism Database, we measured the number of terrorist attacks and deaths from terrorism in those 18 countries each year.

Next, we used statistical analysis to compare the rates of terrorism that these countries experienced in years before and after adopting armed drones. Our analysis accounts for other factors that could mask the true relationship between armed drones and terrorism, such as regime type, periods of civil war, GDP per capita, US counterterrorism aid, and terrorism in neighboring countries.

Yes, armed drones do appear to decrease terrorism

Across statistical tests, we find evidence that armed drones decrease terrorism. According to our main analysis, obtaining armed drones leads to about six fewer terrorist attacks and 31 fewer deaths from terrorism annually. This translates to a 35 percent reduction in attacks and a 75 percent decrease in fatalities per year. However, these numbers fluctuate based on various modeling choices — which means we are more confident that drones decrease terrorism than we are about the specific amount by which it declines.

Most broadly, our study suggests that there is indeed a compelling counterterrorism rationale for utilizing armed drones to enhance national security. Although armed drone operations can be costly — for example, by causing civilian casualties — our findings strengthen the case that the benefits exceed the costs.

However, the decision of when, how or even whether to employ armed drones remains a difficult one. For example, drone strikes specifically — and leadership targeting, more generally — may be less effective depending on the particular terrorist group's organizational structure and ideology. Drones are by no means a panacea and may not always be a net positive for the world.

For the United States, our study implies that a strategy to combat terrorism "from over the horizon" using long-range technology such as drones could be effective. Skeptics of this strategy point out that withdrawing from Afghanistan makes conducting drone operations more challenging. As political scientists Sarah Kreps and Paul Lushenko wrote here at TMC last year, successful drone strikes require accurate and timely intelligence to identify targets, and this information can be difficult to collect from a distance. Yet the successful drone strike that killed Zawahiri demonstrates that the United States can conduct over-the-horizon operations to combat terrorism, even after withdrawing from Afghanistan.

For the world more generally, our results indicate that the military utility of armed drones makes acquiring this technology attractive to other countries. That means we should expect proliferation to continue apace.

Joshua A. Schwartz is a Grand Strategy, Security, and Statecraft Postdoctoral Fellow at the Massachusetts Institute of Technology and the Harvard Kennedy School. Follow him on Twitter @JoshuaASchwartz .

Matthew Fuhrmann is a Professor of Political Science and Presidential Impact Fellow at Texas A&M University. Follow him on Twitter @mcfuhrmann .