At that time, there wasn't a network of connections among all the groups, said Zimmerman, who specializes in the Yemen-based group, Al-Qaida in the Arabian Peninsula, or AQAP, and al-Qaida's affiliate in Somalia, al-Shabab.
"As the network has become more decentralized, it's become much more reliant on these human relationships and the sharing of resources, advice and fighters, which means that you no longer need bin Laden sitting in Pakistan dispersing cash to various affiliates," Zimmerman said. "They have developed their own sources. ... You can't simply pound on part of the network and expect to see results."
Tom Joscelyn, a senior fellow at the Foundation for Defense of Democracies and senior editor of The Long War Journal, a website that tracks how al-Qaida and its affiliates operate around the globe, said he thinks the Bush and Obama administrations mistakenly defined al-Qaida as a top-down pyramid with a hierarchal structure - that "if you sort of lop off the top of the pyramid, the whole thing crumbles."
Al-Qaida leaders have scattered to other parts of the world, he said, noting that Al-Qaida in the Arabian Peninsula is headed by a former aide to bin Laden, who is now general manager of al-Qaida globally.
More recently, the Treasury Department penalized a senior al-Qaida operative on al-Qaida's military committee who relocated from Pakistan to Syria and is involved with a group plotting against Western targets, he said. U.S. officials have tracked communication traffic going back and forth between Syria and Pakistan and Afghanistan, he said.
"This shows, to my mind, that we're not dealing with this sort of discrete core entity in Pakistan and Afghanistan that can be droned to death, but in fact an international network that poses a lot graver challenges," Joscelyn said.
While Obama is keen to burnish his legacy as a president who ended U.S. involvement in the wars in Afghanistan and Iraq and killed bin Laden, even he has softened his rhetoric on terrorism.
Two years ago, on a trip to Afghanistan, Obama said, "The goal that I set - to defeat al-Qaida, and deny it a chance to rebuild - is within reach."
His administration's most recent terrorism report, released by the State Department in late April, uses a less definitive voice.
"The al-Qaida core's vastly reduced influence became far more evident in 2013," the report said. "Al-Qaida leader (Ayman) al-Zawahiri was rebuffed in his attempts to mediate a dispute among al-Qaida affiliates operating in Syria, with the Islamic State of Iraq and the Levant publicly dissociating their group from al-Qaida."
Michael Sheehan, a terrorism expert at the U.S. Military Academy at West Point, New York, said the top two groups he fears could attack the U.S. are "al-Qaida central" in Afghanistan and Pakistan and AQAP, which has attempted several attacks on the United States, including a failed airline bombing on Christmas Day in 2009 and the attempted bombing of U.S.-bound cargo planes in October 2010.
"The other organizations right now - although potentially very, very problematic - are currently focused on the local fight," said Sheehan, the Obama administration's former assistant undersecretary of defense for special operations and low-intensity conflict. "Whether eventually they shift to Europe first, then the U.S., we'll see. But certainly a potential is there."
