Al Qaeda supremo Osama bin Laden may have believed an unseen force was protecting him. He survived close calls fighting the Soviet troops in Afghanistan but was never in as much peril as when sheltering in the caves of Tora Bora with the core of the Al Qaeda leadership, three months after 9/11. By December 2001, the Taliban's Islamic Emirate in Afghanistan lay in tatters, attacked by vengeful B-52 bombers from the air and the Northern Alliance and US special forces on the ground. Bin Laden was caught between the hammer of US special forces and their Afghan allies fighting their way up towards his mountain cave redoubt and the anvil of Pakistani frontier troops blocking his escape. When it seemed like all was lost for the terrorist chief, providence once again came to his assistance in the form of another sensational terror attack. This one, carried out by Pakistani terrorists against India's Parliament in New Delhi on December 11, 2001. The attack saw India deploy its army along the border threatening Pakistan with war. The Pakistani anvil disappeared. An escape route opened up for the terrorist chief and his followers who would live to fight another day. The attack on India's Parliament, it seems, was no coincidence.

November 25, 2001, Tora Bora

Abu Musab al-Zarqawi and his fighters reached Osama high up in the White Mountains, exhausted from the climb and carrying the extra baggage of distressing news. The bin Laden family convoy had been ambushed at the border. Like so many other Arabs, their driver had been hog-tied with electrical wire and turned over to the Americans for a $5,000 bounty, while their chaperone, Osama's driver Salim Hamdan, and the Saudi husband of Osama's daughter Fatima, were "missing, presumed dead".

Horrified, Osama sent a courier back down to Kandahar as he set Zarqawi and his men to work fortifying bunkers. The Jordanian hard man, who some years later would go on to form ISIS, was still far from full strength. Now, he lay on his side in a cot inside the lip of a cave, directing his deputies, and corralling the fighters, who took up position high in the cliff faces, hefted sandbags, shored up cave walls, and stockpiled munitions and weapons for the coming showdown.

Inside one of the last Al Qaeda safehouses, 9/11 architect Khalid Shaikh Mohammad, jehad logistician Abu Zubaydah, military commander Saif al-Adel, and Osama bin Laden's personal spiritual adviser, Mahfouz Ibn El Waleed made a decision to send all remaining Arab families to Pakistan. Those who made it across would be guided by Lashkar-e-Taiba, the ISI-backed 'Army of the Pure'.

This group and many others like it represented one of Pakistan's most enduring contradictions-a state-sponsored Islamist terror network that shared a public platform with senior generals and intelligence chiefs who made vociferous denials of any connections between them....

November 29, 2001, Jalalabad

In recent days, as it became obvious to everyone in the Bush White House that Tora Bora was to be the scene of the final showdown, Jalalabad had filled up with reporters, CIA operatives, hulking Army Rangers, and Delta Force operatives, their commanders bedding in and seeking alliances with Afghan warlords still loyal to the Taliban's main enemy, the Northern Alliance. When the time came to attack, the plan was for the Afghans to lead the way so as to have as few American groundcasualties as possible.

Authors Adrian Levy and Cathy Scott Clark. Authors Adrian Levy and Cathy Scott Clark.

On a map, it was little more than a mile from the foothills of the White Mountains to the first tier of Al Qaeda caves, but the snow was thick, the slopes were steep, and, for even the fittest Afghan fighters, it was an icy three-hour climb. Despite these disadvantages, Bush and his military commander, General Tommy Franks, were confident that spending $70 million on ground support backed up by US airpower would win them one of the biggest "bargains in history".

From his vantage point in Islamabad, the CIA station chief Robert Grenier hoped they were right, but all of his years in a greasy business that-like rally driving-saw traction come and go told him not to bank on it.

Overheads, radio intercepts, and interrogations all pointed to a significant number of jehadisdug in on ragged peaks that topped fifteen thousand feet and ran along an east-west axis, defining a portion of the Durand Line that the British had demarcated in 1893.

Grenier knew this area incorporated some of the most inhospitable terrain in the world, especially around Tora Bora where Osama had superior knowledge. The CIA maps showed that over the years the area had been modified by tunnel rats who had hacked into the quartz and feldspar.

Some caverns were supposedly 350 feet deep and fitted with ventilation exhausts, secret exits, and booby-trapped entrances-and even a hydroelectric power plant. Locked inside the CIA station, Grenier studied large-scale American flight charts that had been drawn up at the height of the Cold War. He had given some to ISI analytic chief General JavedAlam Khan in the days when they had been living in each other's laps, as the ISI's own maps predated the 1947 partition of India and Pakistan. Without consistent satellite time, other than what was secretly lent them by the Chinese, the ISI, the Pakistan army, and the Frontier Corps, a paramilitary force staffed by tribal recruits, were reliant on Pashtun guides and Colonial-era gazetteers to hunt and kill along the border. "The blind leading the blind."

Studying the maps now, Grenier could see something disturbing. As US forces converged on Jalalabad, if Osama's fighters were pressed they were likely to spill across the snow line and vanish into Pakistan's Tribal Areas. He knew that this deeply conservative region was not governed by Islamabad but through tribal customs that demanded residents lend protection to any guests irrespective of the situation. Here an entire army could arrive and vanish under the silk handkerchief called Pashtunwali.

They had to act. He called on General Khan. Officially, homegrown paramilitary units raised from Pashtun villages maintained security in the tribal belt, Khan explained. They were led on and off by regular officers from the army and the Frontier Corps. They were disciplined and could be counted on to stand and fight, unless "a fortuitous war with India" forced the command to reassign all of the armed forces, he joked.

December 3, 2001, Tora Bora

Caves meant many things to Osama. Key among them, aside from the thick, bomb-resistant walls and the cool shade in scorching summer, was their symbolic value. The Prophet had first encountered the angel Gabriel in a cave in Mecca, and Osama once astonished his Mauritanian spiritual adviser Mahfouz by telling him that his-Osama's-presence in Tora Bora gave credence to the oft repeated story that he was the Mahdi (Islam's messiah). "A cave is the last pure place on this earth," Osama had said to the Mauritanian, somewhere to retreat from society.

Now, as mujahideen scrambled up the slopes and into grave-like trenches, Osama took out his Yaesu VHF radio set, reminding anyone listening of the great victory at Jaji. "The trench is your gateway to heaven," he declared, ordering fighters to observe the Ramzan fast even in the heat of battle. At his side were sons Othman and Mohammed; his deputy Dr. Ayman Al-Zawahiri; and his accidental spokesman, Sulaiman Abu Ghaith, who had awayfronted most of Al Qaeda's post 9/11 videos. Zarqawi's men were bedded in all around the peaks. Tough it out, the Jordanian ordered. The Americans were too weak and feckless to ever reach the summit.

Over the border in Pakistan, Grenier, who was by now back in his office at the US embassy, heard that the US side had caught a break. Local men Osama had employed to build the mountain complex had been bought up by the CIA to act as guides, leading targeting and reconnaissance specialists toward a ridgeline from which they could look directly into the Melawa Valley and Al Qaeda's forward operating base.

Astonished, Grenier listened to field reports that described Osama's command posts, vehicles, and stone outbuildings. Dozens of Al Qaeda fighters were spotted in machine-gun nests and antiaircraft positions that had been built into the vertiginous cliffs. The reconnaissance and targeting mission flashed coordinates back to CENTCOM, and the first missiles screeched in. Grenier followed cables about the build-up anxiously, and by the afternoon he could see from the reports that bombers and jets crisscrossed the cloudless sky above Tora Bora, filling the valley with vapours and smoke. Above them, inside his subterranean operations centre, Osama called on his men to be patient, while he sipped tea and ate dates.

On the Pakistan side, six Pakistan Army battalions, freshly kitted out with US materiel, were climbing into position high above Parachinar under the command of Lt. General Ali Jan Aurakzai, the recalcitrant commander of IX Corps. A Pashtun from the Orakzai tribal agency, the general had overall responsibility for the entire northwest of Pakistan. His six thousand men ascended, hand over fist, toward Al Qaeda positions.

December 4, 2001, Melawa Valley, Afghanistan

Believing that a retreat into Pakistan was now imminent, Grenier, who was watching events unfold from Islamabad, consulted his charts and sent a formal request for a battalion of US Rangers to be dropped into position behind the Al Qaeda lines, just to make sure the blocking job was done right. General Tommy Franks refused. They were not going to make the same mistake as the Soviets, he said, deploying huge numbers of US forces that could be drawn into a mountaintop trap. The Pakistanis would do the job for them, acting as the catcher on the high slopes and a beater down in the valley of Parachinar. Lt. General Aurakzai had their back. Osama was surrounded.

Grenier doubted anyone could secure the passes out of the White Mountains and was frustrated that the US military and Bush officials did not press home their distinct advantage. After he recommended that the CIA team on the ground advance, Afghan villagers were dispatched up toward Al Qaeda's position with GPS devices concealed inside food parcels.

One excited man returned, adamant that he had seen Osama bin Laden, a teenage boy who could be one of his sons, and Dr. al-Zawahiri in a cave at fourteen thousand feet. The coordinates were passed back to CENTCOM with a request to send in a BLU-82 Daisy Cutter. The fifteen-thousand pound bomb was designed to explode with intimidating power above the ground, scything a landing strip inside a forest in a split second.

Everything was melting and burning or crumbling. One plaintive voice caught a signals operator's attention: "Father is trying to break through the siege line." Was this code, or had one of Osama's sons radioed through sensitive information on an open line? Working to lock down the signal, the CIA believed it had pinpointed Osama's location to within thirty feet-the closest American forces had ever come to the Al Qaeda leader.

On the night of December 10, Osama reached for his radio set. "What should we do?" he asked the airwaves plaintively. On the morning of December 12, his deputy commander Ibn Sheikh made radio contact with a US-allied Afghan warlord and offered a ceasefire so that Al Qaeda could negotiate his a surrender. They agreed to talk again at 4 pm shortly before the ceasefire expired. Ibn Sheikh called through, asking for an extension until 8 am the next morning, explaining: "We need to have a meeting with our guys." The US side was not sold on the idea but General Franks agreed, overruling the doubters, even though Delta operatives were straining to enter the Tora Bora caves. "Why take your foot off?" an incandescent Grenier fumed in Islamabad.

On the morning of December 13, the 8 am deadline passed without any further communication. Later that afternoon in Islamabad, unscheduled troop activities on the Pakistan side suddenly grabbed the CIA station chief's attention. Without any explanation, Lieutenant General Aurakzaiappeared to be moving his soldiers off the White Mountains. Grenier radioed to check. It was absolutely happening. The troops had been reassigned to Pakistan's eastern borders, with instructions, intercepted by the CIA, to complete the manoeuvre "within three hours." When he tried to get through to Aurakzai and to General Khan, there was no response.

December 13, 2001, 11:45 a.m. , New Delhi, India

Shortly before noon India Standard Time, militants from Jaish-e-Mohammed, the Pakistani jehad outfit run by Masood Azhar and nurtured by the ISI, attacked the main Parliament building in New Delhi, a brazen assault that left 12 men dead and both nations eyeballing each other. Almost immediately, India started deploying soldiers on its border with Pakistan, prompting Major General Musharraf to issue orders to meet them head-on, rerouting Lieutenant General Aurakzai's forces to face them down. "Two active borders are something one would never wish," said Major General Rashid Qureshi, Musharraf's spokesman.

Only a war could come between Aurakzai and the White Mountains,General Khan had warned Robert Grenier weeks earlier. And now Masood Azhar'smujahideen had launched a humiliating assault that brought India and Pakistan closer to fighting-with nuclear weapons-than at any other time since 1999.

Could Jaish have attacked the Indian Parliament without their sponsors in the ISI knowing about it? Grenier fumed. Was the timing some kind of terrible coincidence? It was difficult not to see this as a deliberate ruse to allow Osama bin Laden to escape from Tora Bora into Pakistan. When Wendy Chamberlin, the US ambassador to Pakistan, finally got to speak to Musharraf about the issue, all he would say was that intelligence was a "dirty business". The military combine in Pakistan was like magma, a molten cauldron of minerals and impurities that occasionally formed a crust and was liable to leak out of unseen vents when the pressure became irresistible.

December 14, 2001, Tora Bora

A radio operator caught a snatch of an all-too-familiar voice speaking Arabic. "The time is now!" Osama declared. "Arm your women and children." The messages that followed threw light on the unfolding scene on the mountaintop. Al Qaeda was rallying, sending out small scouting teams to test the viability of the trails to their rear. Quickly, they returned and reported "no resistance". Next came an apology "to all of his fighters," Osama sending admiration and regrets "for getting them trapped and pounded by American airstrikes". Afterward, the thrum of collective prayer filled the airwaves.

In his mind's eye, Grenier imagined the rugged hills above Parachinar. Now instead of a trip wire of forces paid for by the United States, the back door out of Tora Bora was flapping wide open, and through it would stride an exile.