I've never met anybody who thought before 911 that a large-scale terrorist attack on U.S. soil by foreigners was "inconceivable. " I know a lot of people (including me) who thought before 911 that a large-scale terrorist attack on U.S. soil by foreigners was highly unlikely, because those who might otherwise have launched terrorist strikes on the US were deterred from doing so. States and para-states keep the terrorist groups they sponsor, harbor, and supply on a reasonably tight leash. Our intelligence services are good enough to be able to identify with high probability a substantial chunk of the sponsors, harborers, and suppliers of a large-scale terrorist attack. And the U.S. government is capable of swift and massive retaliation against such a sponsoring state or para-state. Thus the elementary logic of deterrence seemed to suggest that every state and para-state would be trying hard to keep their terrorists' leashes short enough to prevent something like the 911 terror bombing of the World Trade Center.
Of course, it happened: deterrence failed. Deterrence can fail for any of five reasons: (1) The terrorists may slip their leash. (2) The sponsoring state or para-state may believe that it can evade being identified. (3) The sponsoring state or para-state may not believe that U.S. retaliation may be massive. (4) The sponsoring state or para-state may think that Allah's strong right hand will defend them against massive retaliation. (5) The sponsoring state or para-state may welcome U.S. retaliation because it has some other, deeper plan of which U.S. retaliation is a welcome part.
It is still not clear to me whether the 911 atrocity took place because of (1) or (4). But it seems to me highly likely that, looking forward, the chances of another atrocity on American soil like 911 are less now than they were on September 10: Every government sponsoring, harboring, and supplying terrorists is now checking the length of the leashes they hold. The U.S. intelligence agencies and the U.S. military appear to have performed reasonably well in identifying responsibility and carrying out retaliation. Allah's strong right hand has not kept the Taliban in power in Kabul, and as Osama bin Laden said, "people like a strong horse more than they like a weak one."
Add to these considerations the domestic consequences of 911 inside the United States: the expansion of the National Security State, the granting of carte blanche to the executive branch in conducting counter-terrorism operations--whether intelligence-gathering or military--the erosion of domestic civil liberties as the security screws are tightened, and so forth, and it seems very clear that U.S. global politico-military hegemony has been substantially strengthened by 911.
Brad DeLong