How could the attacks on 9-11-01 been prevented?

How could we have warned everyone in the Twin Towers in New York City and the Pentagon in Washington D.C.? How could we have stopped the al Quadea from hijacking those planes? I don't remember that day because I was only five years old, I'm still learnin'. Who here remembers that day clearly? Like in the back of your mind? How could we? Why did that horrible Devil let this happen to us? To America? The Twin Towers were part of us and Bin Laden made that useless plan to the al Quadea to hijack those planes and crash them. Why did the Twin Towers fall? I know their beams weren't strong enough, but why? Why?