The 19 men involved with the 9/11 attacks, the USS Cole attack and even the attacks on US embassies in Africa were motivated by the US having troops stationed in the holy land of Saudi Arabia. The troops in SA were there to enforce the No Fly Zone and disarmament of Iraq after the first Gulf war.
If you think through what happened after 9/11, the US took care of this problem right away...by attacking Iraq and moving our troops out of SA. If you want the real motivation for the second Iraq war, you need to look no further than that. The House of Saud was about to be overthrown by religious zealots who were agitated by foreigners, especially female foreign soldiers, tramping around the same desert that was home to Mecca and Medina. Bush was already making plans to invade Iraq before 9/11 because of the growing Saudi instability.
Of course, the US cannot shout this out the world, as declaring the largest oil producing country in the world, and the de facto leader of OPEC, an unstable monarchy about to be overthrown by religious zealots whom plan on creating a worldwide Caliphate isn't really the best thing to say. But look at how quickly stories about the attacks on foreigners in Saudi Arabia left the news stream once the invasion of Iraq began. The attacks are still on going, but the newsworthiness of these stories has been downgraded.