Where was our breakdown?
The breakdown was allowing the hijackers to stay in this country after their visas had expired. Or it could have been allowing them into this country in the first place.
, and how so? Who was behind it,
Bin Laden and his Al Qaeda organization was behind it.
and how'd they manage it?
They outsmarted the security systems that were in place at the time. I think all these answers are pretty well documented.