Psychology of the widespread reaction

The psychological damage caused by 9/11 had a profound impact on politics and foreign affairs in the United States, no doubt about it, and al Qaeda knew this better than anyone. After the Cold War there was an overall sense of relief, the Soviet Union disintegrated and it seemed as if there was no one big enough to take on the United States, but there was, and they weren't even that big, especially when compared to the Soviet Union. All it took was 19 men to cause horror, confusion, and fear even greater than what was felt during the Cold War, because the war was now going on in american soil. The fact that it was only 19 men is, to say the least, humiliating. How can the world's biggest superpower fail to stop less than 20 men from creating havoc and massive destruction?
This feeling triggered the beginning of the world as we know it today, everyone agrees that the horrific events of that day should never happen again. From the United States' particular point of view, it's also the humiliation that must be avoided, which also explains the vengeance streak that followed September 11, 2001. This was just what al Qaeda wanted, the US was forced into the middle east after receiving an attack where it hurts the most: national pride.