Was the success of the terrorists on 9/11 primarily a failure of imagination, a result of our inability to envision that terrorists would hijack planes and crash them into buildings? Fred Kaplan, writing in Slate, argues that's not the case:

As early as 1995, Abdul Hakim Murad told Philippine authorities that he and Ramzi Yousef, who was arrested for his role in the 1993 bombing of the World Trade Center, had planned to fly an airplane into CIA headquarters. This wasn't dismissed as a crazy idea. The year before, a group of Algerians actually had hijacked a plane in France with the intention of crashing it into the Eiffel Tower.

In September 1998, a U.S. consulate in East Asia was warned about an impending al-Qaida plot to fly an explosives-laden airplane into an American city.

Around the same time, Richard Clarke, the White House counterterrorism chief, conducted an exercise in which terrorists commandeered a Learjet, loaded it with bombs, and flew it into a target in Washington, D.C. Clarke asked Pentagon officials what they could do to stop such a threat. They answered they could scramble jet fighters, but they would need authority from the president to shoot the plane down. The exercise went no further.

On Dec. 4, 1998, the President's Daily Brief by the CIA warned that "bin Laden and his allies are preparing for an attack in the US, including an aircraft hijacking" to compel the freeing of those responsible and imprisoned for the 1993 bombing of the World Trade Center.

The North American Aerospace Defense command also conducted an exercise to counter a terrorist attack involving smashing an airplane into a building (though the scenario assumed the plane would be coming from overseas).

Quite independently, in August 1999, the Federal Aviation Administration's intelligence branch warned of a possible "suicide hijacking operation" by Osama Bin Laden.

On May 1, 2001, the Federal Aviation Administration issued a circular to airliners, informing them of intelligence reports about a possible terrorist hijacking.
On June 22, 2001, the CIA notified its station chiefs about an al-Qaida plot to attack American cities with planes.

All of these scenario-spins (plus several others, similarly spelled out in various blue-ribbon commissions) preceded the infamous President's Daily Brief of Aug. 6, 2001, which warned George W. Bush, "Bin Laden Determined to Strike in US."

Why did Sept. 11 happen then? The report of the 9/11 commission argues that the root cause lies in the failure of the intelligence community to coordinate anti-terrorism activities. This manifested itself in many ways; the CIA did not share the names of suspected Al Qaeda members with the FBI - if it had, two of the 9/11 hijackers would have rang alarm bells when they bought their tickets; the CIA could not persuade other intelligence agencies to focus on Bin Laden, because it did not control their funding; the FBI's domestic counterterrorism plans of the late 90s were similarly starved of cash; finally, counterterrorism was generally not discussed at the highest levels of government, promoting further uncoordination among the agencies.

To remedy this, the commission proposes taking away most of the powers of the director of the CIA and giving them to a National Intelligence Director -- someone who controlled the budgets and appointments in all intelligence agencies. Its a good idea: as long as different intelligence branches have different budgetary sources, they will ultimately represent the federal agency that funds them - a recipe for policy clashes and hampered coordination.

Unifying intelligence powers in this way is politically tricky: many different agencies stand to lose power because of it and are likely to oppose it. President Bush, for one, has come out against it. But then there is hope: following George Tenet's recent resignation, we have no CIA director at the moment so the CIA may not be able to oppose the measure effectively; moreover, lets not forget that the Department of Homeland Security, the brainchild of Senator Lieberman, was fought tooth and nail by the Bush administration until it was clear its passage was inevitable.