But, having said that, US "prevention" efforts generally have been less than "shock and awe"-inspiring. Our "state building" operations in Iraq and Astan were certainly intended to prevent AQ expansion. Now, thousands of lives and just south of $ 2 trillion later, we have basically nada - those two "state building" efforts have been the "spectacular failures".
A subsidiary issue for that separate discussion is how far afield does the US go in preventing the expansion of AQ "franchises" (as opposed to hitting AQ Base). In short, the feasibility and the costs of mounting those operations (whatever they might be) may well be prohibitive with respect to local "AQ" groups.
In retrospect from 9/11, the US has been successful in small direct actions and drone strikes against AQ Base and the leadership of closely-tied franchises on an international scale. The US has also been successful within CONUS in prosecuting hundreds of AQ inspired local terrs - with only one shootout (Detroit MI) that I know of.
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