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You can't really blame Bush for 9/11 but you can blame him for what happened afterwards.
Something like 9/11 had been brewing for many years before Bush. America - under various presidents, including Clinton - had not exactly covered itself in glory with its foreign policy, particularly towards the Arab world (Israel/Palestine). These policies acted as a recruiting sergeant for Islamic jihadists and it was inevitable that, one day, they were going to get through.
And I think Karen Greenberg is right when she talks about the damage done to America's image because of Guantanamo/Abu Ghraib/torture. This damage cannot simply be undone, even if Guantanamo closed tomorrow.
It was chilling to see an advanced democracy like the US regressing to a less civilized time where torture was permissible. This is the wrong direction to be going in.
Obama must correct this but, as Greenberg says, something nasty's been let out of the box that you can't easily stuff back in again.