He certainly isn't doing everything to prevent another one. Giving CIA interrogation data to al-Qaeda isn't exactly protecting this country.
Every President has done that?
Illegal aliens have killed, raped and harmed more Americans than AQ has. There is no War on Illegals, only because the deaths and destruction is more drawn out.
But, we as a society has decided to live with death in exchanged for freedom and liberty. Every year tens of thousand of Americans are slaughtered on the road because of driving accidents and drunk drivers. That's life, and we have decided to live with it.
There will always be someone who wants to attack and hurt us. Simple odds, the fact that so few have been able to do it post 9/11 implies the threat is small or the security is so good. Your pick. The fact that we continue to keep a open border when we have the ability to militarize it, during a WOT, speaks for itself, either we are willing to allow the deaths of Americans for political gain, or we are willing to allow foreigners kill our citizens, in exchange for their workforce.