Why did the Bush Administration allow Al Qaeda to achieve its main objective(Details) so easily ?

‘Over the years, bin Laden and his fellow terrorists launched numerous strikes against U.S. targets with the stated design of getting American forces out of Saudi Arabia – including the twin embassy bombings in East Africa; the attack on the USS Cole in Yemen, and, eventually, 9/11.’ http://www.cnn.com/2003/US/04/29/wbr.saudi.base/index.html ‘The United States has said that virtually [...]