And yet progress in degrading (let alone eradicating) groups such as the Islamic State in Iraq and Syria (ISIS), al Qaeda, Al-Shabaab and the Taliban is fiendishly difficult to measure.
Is there a key leader whose demise would deal a hammer blow? Does so-called "leadership decapitation"
even work as a strategy? Or do Western counterterrorism agencies face an indefinite process of crossing names from a never-diminishing list? Are there just too many groups in too many places to combat as we witness the emergence of a new generation of terrorist leaders whose significance is yet to be grasped?
The most important terror figures to have been killed in the last five years include Osama bin Laden and Anwar al-Awlaki, a Yemeni-American cleric. Both were charismatic figures within al Qaeda. The death of bin Laden in 2011 was symbolically important, a morale-booster to the U.S. after nearly a decade of fruitless searching. Most analysts concur that his successor, Ayman al-Zawahiri, commands neither the respect nor loyalty that bin Laden did, and al Qaeda is ultimately weaker for that.
So killing bin Laden mattered. But in terms of operational control and even strategic direction he was a shadow of his former self when finally tracked down to Abbottabad in Pakistan. His death may even have led to over-confidence that the U.S. was making rapid progress from "degrade" toward "destroy" in its battle against al Qaeda.
Months after bin Laden's death, amid an intensive drone campaign, then U.S. Secretary of Defense Leon Panetta said
he thought "at most, we're looking at maybe 50 to 100, maybe less" al Qaeda operatives in Afghanistan.
Two years later, Panetta offered a more nuanced verdict:
"We have slowed the primary cancer -- but we know that the cancer has metastasized to other parts of the global body."
The killing of Awlaki in a drone strike in September 2011 deprived al Qaeda of its most effective propagandist. Awlaki was fluent in English and his eloquence in calling on Muslims in the West to join jihad highly influential. But even from beyond the grave, the cleric's online lectures and sermons have exerted a powerful influence on would-be jihadists, including the Boston bombers.
A study by Fordham Law School's Center on National Security
finds that almost a quarter of the terror suspects prosecuted in the U.S. in recent years were influenced by Awlaki.
Similarly, the killing of Abu Musab al-Zarqawi in 2006
in a U.S. airstrike was legitimately seen at the time as a body-blow to the group he led -- al Qaeda in Iraq. But by then he had laid the foundations of a group that would eventually morph into and inspire ISIS.
Analysts and historians of terrorism are divided on the effectiveness of "leadership decapitation" -- an approach that has been at the heart of the Obama Administration's policy through the widespread use of armed drones. Max Abrahms, who studies terrorism at Northeastern University, says the aversion to putting boots on the ground and the growing technological prowess of drones has made targeted killings the "cornerstone of U.S. counterterrorism strategy."
In theory, he says, it ought to work by degrading the quality of a group's members and thus its threat. But Abrahms told CNN the "strategy has been quite disappointing because the theory behind leadership decapitation overlooks an important point: replacements are seldom more moderate than their former leaders."
In an article to be published in the journal "Terrorism and Political Violence," Abrahms and economics researcher Jochen Mierau argue that militant groups may become even more extreme by shifting their violence from military to civilian targets. So taking out the leaders of a terror group may actually breed greater mayhem, at least in the short term.
Who to target?
One of the difficulties is knowing who really matters in groups whose governing structures are, at best, opaque. Who is destined from a crowded field to emerge as key leaders? In 2003, few would have bet that Zarqawi would soon bring Iraq to the verge of sectarian war while pumping millions of dollars into al Qaeda's treasury.
In 2011, no one imagined that a bland-looking doctoral student in Islamic Studies would three years later declare himself Caliph from a pulpit in Mosul.
Beyond the "sheikhs" and spokesmen, who are the financiers, bomb designers and military planners, the unknown senior operatives? A name that may mean nothing to the outside world could be a crucial figure within al Qaeda or ISIS.
al-Sudani is one example. Al-Sudani was not a household name, but U.S. officials described him as a member of al Qaeda's Shura Council and explosives expert, and also "linked to external attack plotting against the United States." If so, he was a source of expertise and experience that al Qaeda will sorely miss.
Similarly, a Tunisian commander in ISIS, Tariq al-Harzi,
was targeted and killed in an airstrike at the beginning of July, according to U.S. officials.
Whilst not among the senior leadership, al-Harzi was still a key link in the chain as the "emir of suicide bombers" and in the recruitment of Tunisians to ISIS, according to officials in the U.S. and North Africa.
The killings of al-Harzi and al-Sudani, along with that of Nasir al-Wuhayshi,
leader of al Qaeda in the Arabian Peninsula (AQAP), in June, were clearly big "gets." And yet the intelligence on how these groups work, their goals and capabilities, is often sketchy, and that makes identifying, finding and eliminating their key figures more difficult.
Few intelligence officials anticipated ISIS' sudden capture of Fallujah in January 2014, nor its lasting significance. In an interview at the time,
President Obama described those who had raised the ISIS flag over the city as a "JV" (junior varsity) team. His exact words: "I think there is a distinction between the capacity and reach of a bin Laden and a network that is actively planning major terrorist plots against the homeland versus jihadists who are engaged in various local power struggles and disputes, often sectarian."
Even fewer officials predicted the lightning ISIS offensive that seized the Iraqi cities of Mosul and Tikrit around five months later. Asked on the PBS show "Frontline" in May about the fall of Mosul, the Chairman of the Joint Chiefs of Staff, General Martin Dempsey, said there were "several things that surprised us about ISIL ... the degree to which they were able to form their own coalition, both inside of Syria - and inside of northwestern Iraq; the military capability that they exhibited."
Such gaps take time to fill. If a group's military capability and intentions are unknown, its leadership structure is likely to be just as impenetrable. Additionally, ISIS' obsession with secrecy has so far served it well. If you don't know for sure who you've killed, who can you cross off the list?
Organizationally, ISIS appears to have devoted time to "succession planning" -- aware their leaders are the targets of a vast intelligence-led campaign. ISIS has a sophisticated hierarchy
, according to analysts following the group. Military operations are devolved to regional commands, and the group has developed a bureaucracy
that includes Military, Financial and Intelligence Councils, with a Shura Council setting overall policy.
This structure provides redundancy in a way that would mitigate the impact of al-Baghdadi's death. Of course it would be a major achievement to take him off the battlefield, but ISIS would not close down overnight.
The metastasizing described by Panetta is perhaps the core of the problem today. In the fall of 2001, a good chunk of the al Qaeda leadership was holed up in caves in Tora Bora in Afghanistan. Now, the task involves tracking and targeting terrorists from forests in northern Nigeria to the shores of the Mediterranean in Libya and eastwards through Syria and Iraq to the foothills of the Himalayas. And instead of one network, there are two major terror organizations-- ISIS and al Qaeda -- and a bewildering array of loosely connected affiliates.
Intelligence about these groups varies widely. So little is known about how Boko Haram in Nigeria functions, and how (or even if) its leader Abu Bakr Shekau commands the group, that it's very difficult to gauge what would happen were he killed.
Greater cooperation and coordination between groups may also make individual leaders less crucial. ISIS combines individuals who fought in the al Qaeda insurgency against U.S. forces in Iraq with former Iraqi army officers, AQAP and Shabaab have exchanged personnel and other resources, Boko Haram fighters have mixed with al Qaeda in the Islamic Maghreb. This cross-fertilization seems likely to breed resilience and mutual support networks.
Next generation terror
Another factor also suggests the target list of terrorist leaders is constantly being replenished.
Al Qaeda's first major overseas attack, against the U.S. embassies in Kenya and Tanzania, was in 1998 -- nearly 20 years ago. The number of bin Laden lieutenants at large has since shriveled to a handful, but a new generation of terrorists has come of age, with C.V.s that list combat experience in the Iraqi insurgency, Libya, Yemen, Algeria, Somalia, Chechnya and elsewhere. They include astute propagandists, masters of bomb design, financial and IT wizards. Some of them were in kindergarten when the 9/11 attacks happened.
The skill sets have been handed down. Al Qaeda's master bomb-maker, Abu Khabab al-Masri, had tutored dozens of apprentices at his camp in Afghanistan before his death in 2008. His work likely influenced AQAP's chief bomb-maker, Ibrahim al-Asiri,
who is thought to have trained yet more bomb-makers from his hideout in a remote corner of Yemen.
Now comes the grooming by ISIS of the "Cubs of the Caliphate" -- young boys already indoctrinated, recruited as executioners, fighters and suicide bombers.
In a recent video purportedly shot in Raqqa, Syria, one of many boys in uniform warns the "crusader coalition" that it will be destroyed "by the Cubs of the Caliphate, those whom at the end of the Hour [end of the world] will march to the lands of Dabiq," a place in Syria where, according to an Islamic prophecy, an epic battle will finally destroy the Christians.
If the conflict with ISIS' ideology is indeed a generational phenomenon, as senior U.S. officials have suggested, perhaps one of those cubs will be on a target list in years to come.