Why:

Osama bin Laden developed an obsessed mind that was not satisfied to use his too-easily-attained-wealth to achieve greatness by working as part of a constructively-oriented team.  Instead, he needed to direct his wealth that was originally derived through construction of buildings and mosques to destroy what others have built.  Destruction, not construction, is productive to bin Laden.

How:

He was fortunate to find a band of wandering, aimless, under-educated men with potential but needing direction.  Bin Laden gave them the means to serve him while making them believe they were serving their God.

He sat in his cave obsessing over his goal to gain through destruction.  He turned to the only true power his potential followers knew:   Islam.  He took Islam hostage, dissected it, discarded parts that would not serve his destructive needs and perverted what remained to serve, encapsulate and provide a facade for his goals of destruction.

His potential followers, in desperate need of a leader, lacking education, having no hope or understanding of how to ally with Muslims or other successful cultures in the East or the West, yet having the wherewithal to want be successful, abdicated their existences and chose the only course available:  bin Laden.  He made his followers dependent upon him as he played sage.  These binladenites followed his brand of religion -- known to civilized people as terrorism.  This was the only pseudo-productive path defined by bin Laden and his followers accepted him as the means to obtain early release to the promised land.

Bin Laden, in his inverted psychological need to de-construct, that is, destroy, rather than to construct as his family had done to earn its fortune, became obsessed with the grandest example of successful capitalism and freedom, the United States of America.  He focused upon one of the obvious symbols of Western success, the World Trade Centers located in the financial center of the world.  His course was so clear that he had no choice and became obsessed with its destruction.  He must de-construct the United States in order to raise himself on his inverted relative achievement scale.

He joined his emaciated version of Islam with his need to destroy and merged this product with his obsession of destroying the World Trade Centers.

What Was Accomplished:

His followers hijacked four airplanes.  Reports indicate they had plans to include additional strikes using various means that failed due to the efficient and rapid reactions of authorities.  The few successful hijackers executed their tasks from pitifully unprotected airports within an hour of each other.  They turned three of the four airplanes into missiles of destruction but were unsuccessful with the fourth.  In fact, they were defeated on that fourth airplane by a band of civilians so deeply civilized that they instinctively sacrificed themselves to save unknown other civilized people.  The massive destruction the binladenites achieved was accomplished by the technology that went into the airplanes and extremely powerful jet fuel  ---  not anything designed or built by bin Laden or his followers.  For this they took years to train, plan and rehearse.  The stayed in cheap motels, worked menial jobs and lived low-key lives spending relatively little money.

The world has witnessed terrorist bullies hijack planes for years but never before has the task been so simplified, invalidly aggrandized and wrongly characterized as a great and complex accomplishment.  It is not.  In fact, these hijackings were less complex because the hijackers were not required to obtain a tangible ransom, negotiate for something or implement an escape plan.  Those aspects were removed by the simple acts suicide.

The terrorists did accomplish massive destruction and loss of life.  They accomplished this using a deceptively simple and relatively inexpensive plan implemented by relatively few individuals.  These terrorists are dangerous for their willingness to use violence that civilized people have had difficulty identifying early because it is not our arsenal.  They are to be feared for their willing use of asymmetric methods  ---  not their intellect or cunning.  Our enemies are not clever or wise enough to have foreseen the violent reactions they engendered in all civilized people. 

Our reactions will be controlled, precise, total and implemented on multiple fronts over time.

Importantly, the terrorists failed to accomplish their main goals of destroying the United States, capitalism, freedom, and imposing fear upon the free world.  Bin Laden failed to effectively use his followers to advance his cause.  Free civilized people everywhere are today more aligned, cohesive and prepared to destroy the uncivilized than before September 11, 2001.