That enemy, Al-Qaeda, continues to exist, and its leader, Osama bin Laden, remains at large. But in the past three years, the group has not succeeded in launching any new attacks that can compare with 9/11 in scale.
Instead, the war on terrorism is increasingly being defined by bombings and shootings that kill dozens or hundreds of people, but not thousands.
Counterterrorism experts say this is one sign that the war on terrorism is having some success in disrupting Al-Qaeda's operations.
Magnus Ranstorp, director of the Center for the Study of Terrorism and Political Violence at the University of St. Andrews in Scotland, said this success is partly due to better intelligence work.
"I think after 9/11, at least from the United States' perspective, [the war on terrorism] has been quite successful," Ranstorp said. "It has forged a global coalition. We have over 90 countries -- perhaps 100 countries -- that share intelligence information."
Intelligence sharing notably led to the arrests of high-level Al-Qaeda suspects in the United Arab Emirates and Pakistan this year. It has also aided efforts to track and block funding sources for terrorist groups.
But Ranstorp said that terrorist groups are also having some success adapting to the pressure. One way is by making a tactical switch to smaller-scale operations that require less planning and less money.
Here is one sign of the change: The official U.S. investigation into the 9/11 attacks concluded this year that the cost of preparing and mounting those attacks was about $400,000 to $500,000 over two years.
By contrast, Ranstorp said, the operating budget of the Islamist militant group blamed for the March attacks in Spain that killed 191 people was one-tenth of that amount.
"An operation to launch a [small-scale] terrorist attack is very inexpensive. The Madrid cell had an operational budget of about 34,000 euros [$41,000] in their possession," Ranstorp said. "So, we are not talking about a lot of money, and this type of money can be harnessed and generated very easily through lots of different illicit means, with no means for the law enforcement or the intelligence community to be able to detect it."
Experts say that means winning the war on terrorism could require years, even decades, more of the kind of stepped-up security measures, intelligence-gathering efforts, and painstaking police work that have become the hallmarks of the post-9/11 era.
But some observers say those measures alone may not be enough. They say preventing new attacks is just half the challenge. The other half is cutting off the recruitment base for terrorist groups.
Hazem Saghie, a political commentator for the Arabic-language, London-based "Al-Hayat" newspaper, said that states combating terrorism must also fight a war of ideas with organizations like Al-Qaeda.
He said the West must encourage political and economic reforms in the Muslim world to ease the frustrations that drive people to radical organizations.
"The homework [job that needs to be done] is not only hitting terrorism, it is doing something quite positive which helps to sort of widen the middle classes and enlightened ideas," Saghie said.
He added that the West needs to inspire the Muslim world with what he calls a "progressive agenda."
"You have to come with a different, more promising, more progressive agenda to energize those who want their countries and their societies in the Middle East and the Third World in general to become more like the West -- more enlightened, more progressive," Saghie said.
This year, there were signs the war on terrorism may increasingly include such efforts. In June, the leaders of the G-8 -- the world's eight most industrialized countries -- established a Partnership for Progress and a Common Future, supporting political, economic, and social reforms in the broader Middle East and North Africa.
Raymond Kenney of the Conflict and Security Studies Program at the University of Aberdeen in Scotland calls this partnership -- often referred to as the Greater Middle East Initiative -- crucial for winning the war on terrorism.
"The single initiative that I would consider the most important in the coming years would be the Greater Middle East Initiative," Kenney said. "The promotion of good governance, the widening of regional access to education and information, the expanding of economic opportunities, these are all of the highest priority in rooting out the problems at the source of the war in terrorism."
The new initiative includes promoting democracy-building programs; providing microfinancing, or small-scale loans, to help more than 2 million entrepreneurs escape poverty over the next five years; and promoting literacy.
No overall budget has yet been established for the initiative, which includes numerous ongoing projects and will closely involve regional governments.
Some analysts say there is little hope a war of ideas will change the minds of extremists, but it could convince other people not to tolerate or support them.
"The problem is not terrorists," said Benjamin Barber, a government and politics professor at the University of Maryland. "There are only a few crazy terrorists. The problem is a large number of people who are not terrorists but who look the other way or support the terrorists."
"You can't change Osama bin Laden's mind," Barber added, "but you can change the mind of the many who create a safe zone for him. That's the job of cooperation, of multilateralism."
Beyond preventing future attacks and reducing terrorist recruitment, many observers say winning the war on terrorism will also require one further measure. That is, assuring that states combating terrorism do not roll back civil liberties in their efforts to root out terrorist organizations.
Otanazar Aripov, chairman of the Mazlum human rights organization in Uzbekistan, said that when the war on terrorism began, he was hopeful that the struggle would bring Uzbekistan closer to the United States and its democratic values. But he said he also worried that human rights advances could become more difficult as security operations were stepped up.
"Among these expectations, we see now our alarm was more correct," Aripov said. "The repressive policy in Uzbekistan [toward political opposition groups] has not weakened but strengthened."
He accused the government in Tashkent of using its cooperation in the war on terrorism for its own purposes.
"The government of Uzbekistan has used its participation in the antiterror coalition -- the partnership with the United States and the international financial aid -- to amplify its antidemocratic, repressive policy," Aripov said.
The Uzbek government denies it has a repressive policy toward opposition groups.
Counterterrorism expert Ranstorp said winning the war on terrorism means keeping a balance between ensuring that nations have the security they need and preserving the social values cherished by those who live there.
Any imbalance, he noted, is only likely to give rise to new problems in the future.
(RFE/RL's Uzbek Service contributed to this report.)
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