Worst Case Scenario
I have earlier alluded to the issue that Arab governments utilize Islamic terrorists as "politics by other means". I believe that most Arab governments are complicite in international terrorism as a way to overthrow their enemies since they are militarily inept. With this in mind, what would Islamic Terrorists gain by attacking the United States the way they have? They know they will never occupy this land-space thus they can only "snipe" at our infrastructre and people. Granted, their "sniping" is deadly and destructive with no apparent rhyme or reason. But looks can be deceiving. Their conspiracy is well planned with the desired effect that they think will turn history in their direction. I believe their ultimate aim is to wear down the fortitude of the American people to retreat from its current position in the world so as to exploit the vacuum the political paradigm shift would entail. The ghost of Viet Nam past still haunts the American people though we are slowly growing weary of its spectre as well. Therefore, for the Arab they will utilize an option that would never be considered by an American populace inebriated with political correctness to their own demise. Namely, a doomsday device in the form of a biological weapon with the express intent of creating the greatest mass casualty not seen in the Western Hemisphere since the arrival of the Pox vis a vis the Spanish Exploration of the New World beginning in 1492.
The effects would be far more devastating than a nuclear explosion because it would not be contained to one city or locale. The effects of a biological weapon would affect America for generations to come and not at that one moment. The weapon of choice would probably be the bubonic plague or some derivative or perhaps the aptly named "Spanish Influenza". The death toll would be far greater than any explosion because millions of americans across the United States would be affected with debilatating disease that would result in whole regions being quarantined to prevent infection. Commerce between cities would be reduced to such a point that starvation would become a real possiblity for millions of Americans not seen since the days of our colonial foundation.
The economic devastation caused by this disruption would be to much to bear on a Foreign Policy that depends to much on perception and not enough on the procedural demands of the Consitution. The result would be an atrophy of American hedgemony throughout the world with armies and navies being redirected back to the United States for its basic defense of the homeland. The consequences of this action would leave open more possiblities for Arab governments than anything they could have dreamed of by direct military confrontation. Shortly thereafter, other minor and major players would try to fill the void with its far deadlier implications for world war.