The primary rationale used by the members of Al- Qaeda in the attack against the United States seemed to be aimed at a general state of being in regards to the United States' stances taken on issues that are seen as impacting many in the Arab world. The fundamental premise here is to create a zone where U.S. intervention is lessened. For example the United States' presence in the oil producing nations, such as Saudi Arabia, is something that motivates members of al- Qaeda. Along these lines, the United States' perceived support of Israel and its approach to the predominantly Arab populated areas such as the West Bank and Gaza are other sources of anger within the organization. The movement, in my mind, is much more political than it is religious. The use of religion might be used to divert attention from its heavily political aims.
Osama bin Laden and al-Qaeda are Wahhabi Muslims, the most radical branch of Islam, some say, and similar to the Taliban though not identical. They want to turn back the clock to the days of the 8th century and live according to strict Islamic law in the Koran. The direct cause of his attacks on the US stem from our involvement in the Persian Gulf War in 1991.
The presence of more than a half million non-Muslims ("infidels") on the holiest Muslim soil, the birthplace of Mohammed and the religion, was a direct affront to Wahhabi Muslims. The fact that the King of Saudi Arabia invited our troops meant they would declare war on the monarchy of Saudi Arabia too.
The strategy was sound - hit America at the heart of its financial district, the center of the military at the Pentagon, and the White House. This will force America to invade Afghanistan, will damage its economy and confidence, and restrict civil liberties, all of which happened. For an investment of about $150,000, they forced us to spend nearly $1 trillion.
Contrary to a very persistent myth, Iraq and Saddam Hussein had nothing to do with the 9/11 attacks, and that is not the reason we went to war there.
You can argue that the principle of radical Islam that led to these attacks was the principle of jihad, or holy war. There is a lot of dispute about what this term means, but to Al-Qaeda, it seems to mean a war against people who do not follow their version of Islam. According to this idea, they should fight against anyone who is against them. To them, the US was a major opponent because they believe that we are opposed to their religion.
Their strategy is to make a big splash with spectacular attacks that would impress other potential radicals around the world.