When the WTC was constructed all the metal girders that made up the structure/skeleton of the building received a spray-on chemical fire retardant. This was burned off by the tremendous heat of tons of burning aviation fuel and the integrity of the towers was thus fatally weakened. When the NYFD firefighters arrived they believed they had a good chance to rescue the people trapped on the floors above the blaze and the chiefs sent those brave men climbing the stairs to do that. They'd just have to climb the 80-odd floors in full gear in time to carry people out from their offices before they succumbed to the fumes or the flames. But this was not their normal fire. The rescuers got so close. A woman on a frantic call to a 911 operator reported hearing fire fighters reaching her floor. At the same time a structural engineer standing on the street some distance away observed that the towers were beginning to buckle and lean inwards as the compromised metal beams gave way. He sent a message to the NYFD chief who was still directing rescue crews to climb the stairs, telling him that collapse was imminent. You can see on video the very moment the chief gets the message, next moment a huge roar makes him look upwards and the tower is coming down. The NYFD chief didn't have time to issue an evacuation order to the firemen high above; even if he had it's doubtful that New York's bravest would have turned their backs on the souls they had gone in to save.
Bin Laden's terrorists did this. Your last sentence implying that Bush somehow "allowed it to happen" confuses consequence with cause. That's foolish. Yes it "suited" Bush (who I didn't care for) to strike against Bin Laden afterwards but it's absurd to argue that the administration was in on it. Actually it's beyond absurd, it's disgusting.
