The FBI's work on stopping terrorism (including coordination with other agencies) has been nothing short of heroic.
But, eventually, one will slip through. What's important is how we react to it. GW Bush, idiot that he is, reacted irrationally and like a child instead of a calm, measured adult. The result (ISIS, 4000 dead americans, tens of thousands of Americans maimed and psychologically ruined, three trillion dollars lost) was devastating to America. Far more devastating than the attack itself. Hopefully it's either a Democrat or Jeb Bush in office when the next one occurs, because most Republicans are too irrational and bloodthirsty to react in an appropriate fashion to such an event.