Often conspiracy theorists (read uninformed whack jobs) will scream to anyone that listens about how the United States was unable to defend its own headquarters and cities and therefore it must be a conspiracy. This shows a blatant ignorance of history. So why was the United States so vulnerable to attack on 9/11? The reason lies is how we view nuclear weapons. When the United States lost its monopoly on nuclear weapons when the USSR joined them, deterrence became a huge issue that dominates the thinking even today as rouge nations like Iran and North Korea pursue, and have already acquired, weapons of their own. Throughout the Cold War there were two lines of thinking on deterrence. The first was promoted by Thomas Schelling. He argued that in order for deterrence to work, you needed a threat that was left to chance. The mere thought of massive escalation would be enough to deter any potential attack, and every actor in the international system was rational at their core. Schelling promoted the idea of the "balance of terror" which called for no defenses, and left US cities open to attack. If you buy into this line of thinking, this makes perfect sense. He argues that defensive weapons upset the balance of terror and therefore actually would lead to more war as opposed to preventing war. The other line of thinking was by a man named Herman Kahn. He argued that deterrence would only work if your extended deterrence was credible. The way that you got this credibility was through defensive weapons. Now both of these are simplifications of the theories, and if you want to get more in depth I would be happy to do so. Both lines of thinking are logical and have very good points to back them up. In the United States however, it was Schelling's line of thinking that won out. You can see this in the 1972 ABM treaty which basically outlawed defensive weapons and codified the balance of terror idea. It was not until the Bush administration that this line of thinking was changed. However, think about this for a moment. The United States had bought into the idea of no defensive weapons to protect its cities for 30 years. If you look at the size of the NE air defense corridor, it is massive. Since the United States has spent 30 years preparing for no defense, it is not surprising that on 9/11 there was no defense. On that day, only 2 (depending on your source) interceptors were able to get airborne, and the 9/11 Report found that "some" of the interceptors that got up were not even armed. Guess what the plan was? They were going to "bump" the plane in an attempt to bring it down. Bump the plane. It should surprise no one since we bought into the idea that defensive weapons are not needed and spent 30 years ensuring that we did not build any. It was not some conspiracy that resulted in the slow response to 9/11, it was a culmination of 50 years of Cold War policy and deterrence thinking that eliminated the need for defense to ensure that American cities remained vulnerable to Soviet nuclear weapons to ensure that the "balance of terror" would not be upset. Good idea? You be the judge.