Sentence:  What is unbearable, in fact, is the feeling, 13 years after <WSD>9/11</WSD>, that America has been chasing its tail; that, in some whack-a-mole horror show, the quashing of a jihadi enclave here only spurs the sprouting of another there; that the ideology of Al Qaeda is still reverberating through a blocked Arab world whose Sunni-Shia balance (insofar as that went) was upended by the American invasion of Iraq. 
Meaning of the word marked with <WSD> tags:  The attack itself.