Four days before the fall of Kabul in November 2001, Osama bin Laden was still in town. The Al Qaeda leader’s movements before and after September 11 are difficult to trace precisely, but, just prior to the attacks, we know that he appeared in Kandahar and urged his followers to evacuate to safer locations in anticipation of U.S. retaliation. Then, on November 8, he was in Kabul, despite the fact that U.S. forces and their Afghan allies were closing in on the city. That morning, while eating a meal of meat and olives, he gave an interview to Hamid Mir, a Pakistani journalist who was writing his biography. He defended the attacks on New York and Washington, saying, “America and its allies are massacring us in Palestine, Chechnya, Kashmir, and Iraq. The Muslims have the right to attack America in reprisal.” Six months later, when I met Mir in Pakistan, he told me that the Al Qaeda leader had, on that day, appeared to be in remarkably good spirits.

Kabul fell on November 12, and bin Laden, along with other Al Qaeda leaders, fled to Jalalabad, a compact city in eastern Afghanistan surrounded by lush fruit groves. (He was quite familiar with the area, having maintained a compound in a Jalalabad suburb in the 1990s.) Tracking bin Laden closely was Gary Berntsen, a bear-sized CIA officer with a pronounced Long Island accent, who arrived in Kabul on the day it fell. Berntsen had been serving in Latin America on September 11 when he was yanked to run the CIA’s fast-moving ground operations in Afghanistan. It was a perfect job for an operative with a distinctly independent and aggressive style.

By November 14, Berntsen was receiving a stream of intelligence reports from the Northern Alliance that the Al Qaeda leader was in Jalalabad, giving pep talks to an ever-growing caravan of fighters. Berntsen dispatched an eight-man CIA team to the city. To provide them with local guides, he made contact with Hazarat Ali—an Afghan commander, longtime opponent of the Taliban, and nose-picking semi-illiterate. Ali sent three teenaged fighters to escort the U.S. team into Jalalabad, which was now crawling with fleeing Taliban and Al Qaeda fighters.

But bin Laden wasn’t in Jalalabad for long. Following the fall of Kabul, Jalalabad descended into chaos; no one was in charge for at least a week. Abdullah Tabarak, a Moroccan who is alleged to be one of bin Laden’s bodyguards, reportedly told interrogators that, during the month of Ramadan, which began on November 17, bin Laden and his top deputy, Egyptian surgeon Ayman Al Zawahiri, left Jalalabad and headed about 30 miles south. Their destination was Tora Bora, a series of mountain caves near the Pakistani border. Berntsen’s team remained one step behind them, for now.

Tora Bora was not yet a familiar name to many Americans. But what would unfold there over the subsequent days remains, eight years later, the single most consequential battle of the war on terrorism. Presented with an opportunity to kill or capture Al Qaeda’s top leadership just three months after September 11, the United States was instead outmaneuvered by bin Laden, who slipped into Pakistan, largely disappeared from U.S. radar, and slowly began rebuilding his organization.