But, your not getting the point, Al Quida didn't attack the United States for the heck of it, they were retaliating. They had a good reason to attack. Just like you said, your not gonna sit idle when your county is bieng attacked. So thats exactly what they did.. They attacked... heres a Histroy Lesson:
After World War II, most of the Jewish population had died. As Germany was the county they had settled into, it no longer was, and they had no land to call their own. Therefore, no Jewish Nation. After that, there was the discovery of (very valuable) Oil in the middle east (Arab Nations). The United States took note of this, and supplied the Jewish people (Israel) with weapons and such, to wage a war on the weakest Arab nation, which was Palestine.. In hope of an accomplishment of taking over some countries, and Israel to supply the United States with Oil. They succeeded in overthrowing the Palestinians, and continued to battle other countries for more land.
The Wealthiest and most powerfull Arab nation, Syria, took action against this, and helped other Arab nations claim back their land, and they did. Till this day though, Palestine and Israel are still at war, as you should frequently se on the news. Most parts of some Arab nations where blown up to pieces (Including my home town, but that was long ago).. The United States where blamed for this.. The Attack on September the 11th was Revenge, and a warning to the United States.
Thats the simplest way I can put it.. You might want to lok futher into that if you want, but it's a loooong story, including owning of religious land where supposedly Jesus was born (called, "The Holy Land") and Anti-Jewdaism...