On Sept. 11, 2001, the core of al-Qaeda was concentrated in a single city: Karachi, Pakistan.

At a hospital, the accused mastermind of the bombing of the USS Cole was recovering from a tonsillectomy. Nearby, the alleged organizer of the 2002 bombing in Bali, Indonesia, was buying lab equipment for a biological weapons program. And in a safe house, the man who would later describe himself as the intellectual author of the Sept. 11 attacks was with other key al-Qaeda members watching the scenes from New York and Washington unfold on television…

Four days after the Sept. 11 attacks, bin Laden visited a guesthouse in Afghanistan’s Kandahar province. He told the Arab fighters gathered there “to defend Afghanistan against the infidel invaders” and to “fight in the name of Allah.”

It was beginning of a peripatetic three months for bin Laden and Zawahiri.