http://en.wikipedia.org/wiki/Al_qaeda#StrategyStrategy
On March 11, 2005,
http://en.wikipedia.org/wiki/Al-Quds_Al-Arabi">Al-Quds Al-Arabi published extracts from
http://en.wikipedia.org/wiki/Saif_al-Adel">Saif al-Adel's document "Al Quaeda's Strategy to the Year 2020".
http://en.wikipedia.org/wiki/Abdel_Bari_Atwan">Abdel Bari Atwan summarizes this strategy as comprising five stages:
1. Provoke the United States into invading a Muslim country.
2. Incite local resistance to occupying forces.
3. Expand the conflict to neighboring countries, and engage the U.S. in a long war of attrition.
4. Convert Al-Qaeda into an ideology and set of operating principles that can be loosely franchised in other countries without requiring direct command and control, and via these franchises incite attacks against countries allied with the U.S. until they withdraw from the conflict, as happened with the 2004 Madrid train bombings, but which did not have the same effect with the 7 July 2005 London bombings.
5. The U.S. economy will finally collapse under the strain of too many engagements in too many places, similarly to the Soviet war in Afghanistan, Arab regimes supported by the U.S. will collapse, and a Wahhabi Caliphate will be installed across the region.
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I think there hasn't been a major attack inside the U.S. by Al Quaeda since 9-11 because they haven't needed one. They probably figure things are going pretty much according to plan.