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.