But three years later, the international terrorist organization, its associated organizations, and independent groups inspired by its ideology and methods have broadened the battlefield dramatically.
"I saw two very powerful explosions, and ambulances started to arrive, dozens of ambulances, policemen, and firemen," said a witness describing terrorist attacks in Madrid in March, when bombs placed on commuter trains killed 191 people. Investigators subsequently found that one member of the group of Islamic radicals blamed for the attacks had contacts with senior Al-Qaeda members.
The bombings in Spain were the first major attacks in Europe with suspected links to Al-Qaeda and its associated organizations. Later attacks in Central Asia and the Middle East showed that Europe was only one battleground.
In late March and early April, suicide bombers in Uzbekistan launched multiple attacks on police in Bukhara and the capital Tashkent. By the end of a week of bombings and shootings, many in crowded public places, at least 47 people were dead.
Uzbek officials said the militants involved in the attacks were members of fundamentalist Uzbek Islamic groups but that they also had links with Al-Qaeda associates.
"Some of the detained individuals have testified that they were trained in terrorist camps by Arab instructors who had previously trained Al-Qaeda militants," said Uzbek Prosecutor-General Rashid Kadyrov.
The Uzbek government's claims are based on prison confessions from arrested militants and cannot be independently verified.
Tashkent has placed most of the blame for the attacks on Hizb ut-Tahrir, a banned opposition group that seeks the creation of an Islamic state spanning all of Central Asia. The group has denied any links to the attacks.
In late May, gunmen attacked a housing compound for foreigners in the eastern Saudi city of Khobar, killing 22 people. The leader of the Saudi cell of Al-Qaeda claimed responsibility for the raid, saying it was designed to punish Westerners for plundering Muslim resources.
Saudi Arabia's de facto ruler, Crown Prince Abdullah bin Abd al-Aziz, decried the assault: "It's both in the interest of Saudis and the people who live in Saudi Arabia that these incidents don't happen. Nobody with any relation to Islam or any humanity and dignity will give them any mercy at all."
These were just the major incidents of the past year, which also saw numerous attacks in Afghanistan and Iraq and a failed attempt by Al-Qaeda-linked militants to assassinate Pakistani President Pervez Musharraf.
Some analysts say the increasingly global nature of the attacks illustrates how resilient the Al-Qaeda network remains, despite Washington's destruction of its operational base in Afghanistan.
The network has survived largely because thousands of fighters trained at Al-Qaeda camps in Afghanistan during the 1980s and 1990s later returned home to join or create local groups across the Muslim world.
These local groups -- which share a desire to create a utopian Islamic state encompassing most of the Muslim world -- have proven able to mount operations with or without support from Osama bin Laden or his direct lieutenants. Bin Laden himself, meanwhile, remains at large.
The local groups also have proven to be a resource for insurgents attempting to copy Al-Qaeda's successes in their own battles with regional governments.
"Increasingly, [the war on terrorism] is a conflict between states and groups," said Julian Lindley-French, an analyst at the Geneva Center for Security Policy. "There are marginal groups like the Chechens, increasingly linking themselves to Islamic extremist groups in their own struggle against the local state. And that is driving states that often had quite difficult relationships together -- Russia, America and China is an obvious example."
Among key countries now working more closely with the U.S.-led war on terrorism than before are Saudi Arabia and Pakistan. Saudi forces now routinely raid Al-Qaeda safe houses, and police in Pakistan arrested several key Al-Qaeda suspects in what were seen as the most significant setbacks for the organization this year.
Lindley-French said the "globalization" of the war offers new opportunities for states to collectively bring massive power against terrorist groups. But at the same, it makes winning a decisive victory more difficult.
"Clearly, when you have a coalition of states facing such groups, then the power they can bring to bear is enormous," said Lindley-French. "But given that, it is very hard to find any particular center of gravity of these groups. It is very hard to destroy them. And, therefore, I think we are moving away from the period in the immediate aftermath of 9/11 when the U.S., in particular, sought a decisive engagement with what it perceived as the enemy, increasingly toward a policy of a new containment of such groups."
He said containment requires efforts ranging from improving intelligence gathering to looking for ways to address political and economic problems that fuel militancy.
As both terrorist organizations and many states increasingly view the war on terrorism as a global battle, most analysts predict the war could last many years more.
But they also say the strategies the United States and its allies must follow to win are gradually becoming clearer.
More stories on the 9/11 anniversary from RFE/RL:
A Day Filled With Unforgettable Events, Images
Experts Say Winning War on Terrorism Requires Patience, Flexibility
Firefighter's Widow Says, 'We Always Think We Have Tomorrow'
Major Events In Global War On Terrorism In Past Year