No event in recent times has produced as many explanations as the attacks of 9/11 and for this reason it is difficult to clearly state why these attacks took place.
Some of the reasons for the attacks can be traced back to events in the Middle East in the 1990s.
After the 1991 Gulf war, the USA kept troops stationed in Saudi Arabia.
Saudi Arabia is the home of the holiest sites in Islam (Mecca and Medina) and many Muslims were upset at the USA permanent military presence there.
The fatwā called for American troops to get out of Saudi Arabia.
In 1998, another fatwā was signed by bin Laden and others complaining about the American military presence in the Middle East, sanctions against Iraq and American support for Israel.
The fatwā seemed to provide religious authorisation for killing of Americans everywhere.
Osama bin Laden wanted to destroy American influence in all Muslim countries, destroy Israel and overthrow pro-Western governments in Muslim states.
He wanted to use terror to re-instate an Islamic caliphate.