While Teribus maks some fair points, I can't let "It was an attack on western civilisation in its entirety, not solely directed against the United States of America." past.
If 9/11 had one unambiguous aspect it was as an attack on the USA. The World Trade Centre was not the sole target, as supporters of the "attack on civilisation" position imply by neglect.
Power is Political, Financial and Military. The 9/11 attacks (plural) were aimed at the White House (probably), the world Trade Centre and the Pentagon, symbols to all the world of American (not Western)power in the three areas.
The target was unquestionably USA