911 Commission Report…as Presidents of Afghanistan, Pakistan and USA recently dined together:
With all this back-and-forth on who to blame for inaction or wrong action pre-911, everyone should read the 911 Commission Report – the authoritative work on the subject.

On page 63, 64, this is what it says regarding bin Laden’s travel from Sudan to Afghanistan in 1996:

“Though Bin Ladin’s destination was Afghanistan, Pakistan was the nation that held the key to his ability to use Afghanistan as a base from which to revive his ambitious enterprise for war against the United States”
“It is unlikely that bin Ladin could have returned to Afghanistan had Pakistan disapproved. The Pakistani military intelligence service probably had advanced knowledge of his coming, and its officers may have facilitated his travel. During his entire time in Sudan, he had maintained guesthouses and training camps in Pakistan and Afghanistan. These were part of a larger network used by diverse organizations for recruiting and training fighters for Islamic insurgences…” Pakistan intelligence officers reportedly introduced Bin Ladin to Taliban leaders in Kandahar, their main base of power, to aid his reassertion of control over camps near Khowst, out of an apparent hope that he would now expand the camps…”

-9/11 Commission Report

And then there is this prescient statement from page 78:

“The relationship between al Qaeda and Iran demonstrated that Sunni-Shia divisions did not necessarily pose and insurmountable barrier to cooperation in terrorist operations. As will be described in Chapter 7, al Qaeda contacts with Iran continued in ensuing years”

-9/11 Commission Report