It was a calculated and cunning attack and clearly it worked beyond the wildest dreams of the terrorists
There is quite a long background history involved but basically 9/11 was collateral damage in a civil war within the world of political Islam.
On one side there are those, like Bin Laden, who want to install Taliban-style theocracies from Indonesia to Morocco. On the other side there is a silent majority of Muslims who are prepared to deal with the west, who do not see the Taliban as a workable model for modern Islamic states, and who reject violence
Bin Laden's total dominance of al Qaeda meant the organisation was hostage to his strategic vision. His analysis of US foreign policy was based on the US withdrawal from Lebanon in 1983, after the attack on the barracks that killed 241 American servicemen, and from Somalia in 1993 after 18 US soldiers were killed in Mogadishu. From these retreats, Bin Laden concluded that the US was a paper tiger, capable of withstanding only a few strikes before it would withdraw, leaving client regimes in the middle east vulnerable. But the US response to 9/11 was to destroy the Taliban regime and decimate al Qaeda. Although 9/11 was a tactical success for al Qaeda, it actually threatened the organisation's future.
The east Asia, Europe, America, horn of Africa, Yemen, Gulf, and Morocco groups have fallen.