In an Associated Press interview, Brennan said the death of Atiyah Abd al-Rahman in Pakistan's tribal areas last week was a "huge blow" to the group, damaging the network and keeping Al Qaeda's leadership too busy trying to hide to plot new attacks. Al-Rahman reportedly was hit by a CIA drone strike.
In a wide-ranging interview, Brennan credited aggressive U.S. action against militants across the region as the main reason U.S. intelligence has detected no active terror plots before the 10th anniversary of the 9/11 attacks.
The former CIA officer described that as proof that the White House has found the right formula to fight Al Qaeda, by pairing U.S. intelligence and counterterrorist forces with host nations from Pakistan to Iraq to Yemen, fighting beside them or sometimes through them. The goal is to keep Al Qaeda off balance, unable to replace the seasoned terrorists the U.S. campaign is taking out.
"If they're worrying about their security ... they're going to have less time to plot and plan," Brennan said of the militants. "They're going to be constantly looking over their shoulder or up in the air or wherever, and it really has disrupted their operational cadence and ability to carry out attacks."
He pointed to the killing of al-Rahman as an example of how U.S. pressure is degrading the network.
"There's no longer a management grooming program there. They don't stay in place long enough," Brennan said.
Al-Rahman had barely assumed a leadership position since bin Laden's death pushed his deputy, Ayman al-Zawahri, into the top spot. Brennan described al-Rahman as a "workaholic" and an "operational mastermind" who kept Al Qaeda's nodes from Yemen to Europe connected.
"Taking him out of commission is huge," Brennan said. "There's not another bin Laden out there. I don't know if there's another Atiyah Abd al-Rahman out there."
Brennan said the key to keeping another al-Rahman from rising is to keep constant pressure on all locations where Al Qaeda operates, working through host countries to target operatives who "are flowing sometimes back and forth" among Pakistan, Afghanistan, Iraq, Yemen, Somalia and other parts of Africa.
Brennan brushed off some of the major crises in those relationships of late, from Pakistan's strident objections to drone strikes as a continued affront to its sovereignty in the wake of the bin Laden raid, to the revolts across the Mideast that swept from power U.S. counterterror allies in places like Egypt.
He said the relationship with Pakistan is improving.
And he described the Arab revolts as a "speed bump" that only temporarily disrupted cooperation. He said U.S. contacts in Egypt have been able to recover quickly following longtime leader Hosni Mubarak's ouster earlier this year. The counterterrorism relationship with Tunisia, where the so-called Arab Spring movement began, also remains strong, he said.
Brennan said the uprising in Yemen, however, had kept Yemeni forces engaged in a fight for political survival, and had slowed down the fight against arguably the most dangerous bin Laden affiliate, Al Qaeda in the Arabian Peninsula. AQAP, as the affiliate is known, has worked with the rebel tribes to grab large swaths of territory in the south.
The unrest has forced the U.S. to draw down the hundred-plus military and intelligence personnel it had working with Yemeni counterterrorism forces. Those Yemeni forces, led by ailing Yemeni President Ali Abdullah Saleh's sons, have been reluctant to leave the capital unguarded, even when a brigade of conventional Yemeni troops became trapped by Al Qaeda in the Abyan region.
U.S. forces had to air-drop food and water to the embattled unit, which was threatening to surrender. Brennan said the U.S. has since persuaded the Yemenis to send enough forces their way to free them, and he has urged the country's vice president to send more troops into the fight.
"This political tumult is ... leading them to be focused on their positioning for internal political purposes as opposed to doing all they can against AQAP," he said.
Saleh is still recovering in Saudi Arabia, with some 70 percent of his body burned and a lung pierced from an assassination attempt in June, as he was praying in his palace compound.
While Brennan says Saudi Arabia would allow Saleh to return from his temporary medical exile, he repeated the White House's earlier calls for Saleh to stay away and let new elections take place.
"I've told him that I do not believe it's in his interests, Yemen's interests or our interests ... to go back to Yemen," Brennan said.
He called Yemen a "tinderbox" that could erupt into a civil war that Al Qaeda would take advantage of.