Our history of meddling in the internal affairs of other nations, propping up dictators, toppling a government, imposing sanctions and embargoes, supporting Israel, placing US troops on their soil, etc, etc.
What haven't we done to piss these people off? At some point, some will say enough is enough and strike back.
It's called blowback and not unexpected. Our problems are the direct result of an interventionist foreign policy.
No rational nation leaves the fate of its critical interests and allies solely to the good faith of others. Furthermore, that Osama Bin Laden's "letter to America" also demanded that the U.S. turn to Islam, complained about the role women play in American society, and rejected the availability of alcohol ("intoxicants" as Mr. Bin Laden described it), etc.--conditions go far beyond actual or perceived foreign policy grievances--it is clear that Al Qaeda's rationale for attacking the U.S. was at least as much about who/what America is than what its policies have been. And such an assessment does not even get to other material relevant factors e.g. Al Qaeda's ideology and aspirations.
If, however, the U.S. had taken the path of isolationism following the end of World War II, it is difficult to imagine that the U.S. would be in a better economic and national security position than it is today. For starters, the Soviets would have had a free hand to pick off non-communist states one-by-one and it could have leveraged its positions among radical Middle Eastern states to lock up that area's energy resources depriving others of vital resources. The result would have been the Soviet capacity to impose and sustain a chokehold until those who sought an alternative way of life would be forced to capitulate.
In addition, on account of its pursuing a policy of isolationist abdication, the U.S. would lose its allies. After all, if the U.S. were indifferent to the fate of its allies even when they were exposed to mortal danger, a bilateral relationship with the U.S. would make no sense. Consequently, its efforts to enter into and sustain economic, political, or security commitments would become all but impossible given its unreliability.
Finally, the East Asian and Western European economic miracles might never have come to pass. There would have been no Marshall Plan, no security framework that allowed for Western Europe and Japan to focus on economic development, etc. Under such circumstances, even if the Soviets restrained themselves from spreading their totalitarianism beyond the Iron Curtain in the face of a frontier opened by American abdication and the enormous power vacuum that was present in the wake of World War II, perhaps new radical ideologies would have taken root and flourished on the war-scarred Continents.
In reality, policy is never risk free. In fact, nothing in life is risk free. Trade-offs are inescapable.
In the end, no matter how it is packaged, implicit in the claim that "blowback" was solely responsible for the 9/11 terrorist attacks is the notion that isolationism would have produced a different outcome. Against the sweep of human history, the assumption that isolationism would transcend risk pacifying would-be aggressors, taming the worst attributes of human nature/ambition, and producing perfect, perpetual, and prosperous security is without foundation.
