It seems obvious: Killing terrorist leaders should weaken their organizations, depriving those groups of strategic direction and ideological appeal. The death of someone like Abu Muhammad al-Adnani, a senior Islamic State figure reported killed on Tuesday in Syria, should seem like a significant setback for the group.
But scholars have struggled to find evidence that killing leaders is an effective way to dismantle terrorist organizations, instead finding ample evidence that it makes little difference. That research seems to apply especially to the Islamic State, also known as ISIS or ISIL, whose attributes make it resilient to losing even a top figure like Mr. Adnani.
Two features make a terrorist group able to withstand a senior officer’s death, according to research by Jenna Jordan, a Georgia Tech professor and a leading expert on the subject.
The first is popular support. Groups need a steady stream of recruits and a pool of potential new leaders. Support among civilians in areas in which the groups primarily operate also makes them more stable, by broadening support networks and helping them to safely retrench when needed. Leaders are usually killed in or near communities that support them, resulting in those communities rallying behind the terrorist group and against whoever did the killing.
While it might be difficult to imagine that a community would support the Islamic State, the group’s continued control over parts of Syria and Iraq and the recruits flooding in from abroad demonstrate its appeal. Religious groups are even better at absorbing attacks, Professor Jordan found, because their appeal is based on a shared identity that transcends any individual leader.
The second feature is not something usually associated with groups like the Islamic State: bureaucracy. The more a terrorist group resembles a corporate organizational chart — often with administrative, payroll and logistical staff — the more stable it is, and the better able to handle a leader’s death.
Just like any other bureaucracy, such groups have clearly delineated hierarchies, internal rules and divisions of responsibility. That clarity means it is easy to replace a leader with a deputy. It also makes the organization stable: If one cog falls out, the rest of the machine can still function.
For a group as large and complex as the Islamic State, the infrastructure is simply too large for any one person, even a top leader, to make or break its future.
This is why terrorism scholars have repeatedly concluded that killing or capturing terrorist leaders — a strategy known, colorfully, as “decapitation” — does not work.
Robert A. Pape, a University of Chicago professor, wrote in a much-cited 2003 study that Israel and other governments had spent “over 20 years” focused on killing or capturing terrorist leaders and found “meager success.”
“Although decapitation of suicide terrorist organizations can disrupt their operations temporarily, it rarely yields long-term gains,” Professor Pape wrote.
It can, in some cases, even backfire. Governments that engage in targeted killings risk resetting ongoing political negotiations. Daniel Byman, a Brookings Institution scholar who focuses on Israeli counterterrorism, has written that a policy of “decapitation” may have led Palestinian terrorist groups to decentralize, ultimately making them more of a threat.
Still, the research is necessarily fuzzy. As one paper laments, definitive conclusions would require setting up an experimental terrorist group, which “is neither desirable nor feasible.”
That paper, by Patrick B. Johnston, a RAND Corporation researcher, is more supportive of “decapitation” strikes, however. Mr. Johnston found that repeated strikes against a terrorist group can, in some cases, increase the chances of a group’s defeat. But such strikes alone, he found, are not enough.
What these studies share is an acknowledgment that terrorist groups are, in at least some important respects, a political phenomenon. They cannot be fully defeated without addressing their political roots, including whatever local support they enjoy.
Abu Musab Al-Zarqawi, the leader of Al Qaeda’s Iraq branch, was killed in 2006 as Iraqi Sunnis turned against the group in large numbers. His death was the result of his group’s decline, rather than a driver of it. Osama bin Laden’s death, in 2011, similarly came after a decade-long ground war to uproot Al Qaeda from Afghanistan and accompanying efforts in Pakistan.
If killing terrorist leaders does little on its own to defeat terrorist groups, then why do countries like the United States make such frequent use of this strategy? Consider where this strategy has been deployed: Syria, Somalia, Pakistan’s tribal regions and Yemen.
These are places where the United States might believe it has few, if any, options. Targeting terrorist leaders might not make much difference, but it is cheap, it is low risk for the United States (though not always for civilians in the vicinity of strikes), and it allows American leaders to credibly say they are doing something. But there is little evidence that these deaths, whatever their political value in the United States, make much of a difference on the ground.
