Five years ago, the United States was struck by the most devastating terrorist attack in its history. Subsequent investigations indicated that al-Qaida, a radical Islamic group led by Osama bin Laden, was responsible for the terrorist assaults that killed almost 3,000 people.

The attacks of September 11, 2001 launched President Bush's global "war on terror" - a struggle that is still going on five years later.

Pakistan army soldier stands alert, as he monitors Afghan-Pakistan border at Kundigar post

The first target was Afghanistan, where the Taleban government was harboring al-Qaida and Osama bin Laden. A U.S.-led coalition ousted the Taleban, but Osama bin Laden remains at large, believed to be hiding in the rugged terrain between Afghanistan and Pakistan.

American Enterprise Institute terrorism expert Danielle Pletka says al-Qaida has been significantly weakened since the beginning of the Bush administration's "war on terror."

"They are constantly under assault. Their financial lifelines have dried up. Their weaponry has dried up. Anytime your leader is hiding in a cave, it is hard to say that you are in the same strong position you were in when you were living in a mansion," noted Pletka.

Many experts agree that the "war on terror" has been successful in degrading al-Qaida's operational capabilities.

One of those is Brian Jenkins, a leading authority on terrorism working for the RAND Corporation. But he says the U.S. and its allies have not been successful in denting al-Qaida's determination to continue its "jihad," or holy war, against the West.

Ayman al-Zawahiri, is seen in this image made from videotape broadcast by Al-Jazeera television on Thursday, July 27, 2006

"We have not blocked their communications. We have not blunted their message. We have not impeded their recruiting, nor have we prevented them from planning and preparing new terrorist attacks. There have been close to 30 communications from Osama bin Laden himself since 9/11 - a greater number from his lieutenant [Ayman] Al-Zawahiri," said Jenkins. "The fact that they can, despite the security risks involved, still deliver videotapes and audiotapes to television stations, indicates an ability to deliver other things. If they can get a tape to al-Jazeera, they can get a secret message to someone else, and it suggests that it would be premature to write off the center."

Jenkins says since 9/11, al-Qaida has transformed itself into something other than a radical Islamist group.

A supporter holds a poster of Osama bin Laden, during an anti-American rally,  in Islamabad, Pakistan (File photo - July 30, 1999)

"Al-Qaida has transcended its historic organizational skin to become an ideology, and I think it is probably more correct today to speak of the 'jihadist enterprise' which is inspired by al-Qaida's ideology," he continued. "Now that may include the veterans of the original terrorist organization. It includes a new cohort of fighters who are gaining their experience and skills in Afghanistan and Iraq today. It includes affiliated groups in Indonesia, in Egypt, in Algeria, in Saudi Arabia. And it includes those self-radicalizing entities who may not have any organizational connections with the historic al-Qaida, or any center at all, but who self-radicalize and who, on the appeal of al-Qaida's message, turn themselves into weapons."

Jenkins says since al-Qaida is now an ideology, the removal of Osama bin Laden would have less effect on the whole terrorist enterprise now than it would have had four or five years ago.

"The fact that he has been able to survive, the fact that he has formulated this narrative over the past five years, the fact that this ideology has spread via the internet and other means of communications throughout the globe, does suggest that his departure now, while it would have some impact, psychological impact, would not necessarily lead to the demise of the enterprise itself," explained Jenkins.

Given the international scope of the terrorist threat, Jenkins and others believe the "war on terror" will go on for a long time. Experts say a successful outcome will involve a combination of vigilance at home and increased international cooperation.