From The Asia Times:WASHINGTON - New evidence from former United States officials reveals that Osama bin Laden and other al-Qaeda leaders were able to skip Afghanistan for Pakistan unimpeded in the first weeks after September 11, 2001, as the George W Bush administration failed to plan to block their retreat.
Top administration officials instead gave priority to planning for war with Iraq, leaving the United States with not nearly enough troops or strategic airlift capacity to close the large number of possible exit routes through the Afghanistan-Pakistan border area where Bin Laden escaped in late 2001.
Because it had not been directed to plan for that contingency, the US military was also forced to turn down an offer from then Pakistani president Pervez Musharraf in late November 2001 to send 60,000 troops to intercept the al-Qaeda leaders.
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My Comment: The priority of the U.S. Administration after 9/11 was to prevent another attack, and to destroy Al Qaeda's infrastructure in Afghanistan. I cannot speak for the administration, but I am sure that while it would have been a bonus to capture or kill Bin Laden, the priority was to destroy Al Qaeda's organization and their Taliban allies. Which at the time they did with limited resources and manpower.