In South Waziristan the enemy is not just the Taliban and al-Qaeda. It's also the terrain, and the militants know every inch.
For the army it is a process of taking the high ground, then the valleys
For centuries the treacherous peaks and remote valleys have been resistant to outsiders.
During the era of the Raj, British troops had their own name for it - "Hell's Door Knocker."
These days American officials regard it as one of the most dangerous places on earth.
In this barren landscape the Pakistani army is now fighting a largely hidden war. We were given a rare glimpse of the battlefield, where 30,000 troops are trying to flush out an estimated 5,000 to 8,000 militants.
The military gave us a guided tour of areas captured since the launch of operation "Path to Salvation" on 17 October.
The terrain dictates the strategy - first troops have to take the high ground, then the valleys and ravines. We were taken to a series of strategic hilltops, which are now in army hands.
Commanders spoke of fierce resistance from heavily armed militants.
One of the first battles was against a stronghold of Uzbek fighters, in the district of Spin Jamaat. The Uzbeks have a reputation as ferocious warriors, and loyal al-Qaeda soldiers.
"They put up a very good fight," says General Khalid Rabbani, who lead the assault. "They defended every peak and every ridge, and they filled the area with mines and improvised explosive devices."
He said his troops had killed 82 militants, but admitted that hundreds had probably escaped Spin Jamaat before the fighting began.
The BBC's Orla Guerin was granted rare access to report from South Waziristan
In a mud compound in the village of Sherwangai, troops displayed some of the spoils of war.
Alongside the heavy weapons and the hand grenades there were computers - and identity documents.
One passport suggests a militant with links to the 9/11 hijackers may have been here at some point.
We had a brief opportunity to examine the German passport, issued in the name of Said Bahaji.
It was issued in August 2001 and showed an entry to Pakistan early the following month - days before the twin towers of the World Trade Center were attacked.
Bahaji is believed to have lived in Hamburg for eight months with Mohammed Atta, the leader of the 9/11 hijackers.
The appearance of the passport raises a lot of questions - not least is it genuine? For now that is unclear.
The discovery coincided with a visit by the US Secretary of State, Hillary Clinton, in which she publicly questioned Pakistan's efforts to hunt down al-Qaeda.
One UK-based analyst said the timing was not an accident.
"I think it's convenient," says Dr. Sajjan Gohel of the Asia-Pacific Foundation, "especially given that Hilary Clinton seemed to criticise the government and the military for not doing enough to find al-Qaeda."
"The question is whether they allow more scrutiny of the passport, " Dr Gohel added.
The army says it is now being examined by Pakistani intelligence experts, and that this process will "take time".
During our visit the soundtrack of war was playing in the hills around Sherwangai - the dull crack and thump of mortars, punctuated by bursts of machinegun fire.
Questions remain over the authenticity of the Said Bahaji passport
Commanders said troops are advancing steadily - if slowly - and the Taliban are apparently being pushed back.
But one senior officer warned that what looks like a retreat is actually a trap.
"Their main aim is to suck us deeper into forested areas and mountain passes, and cause maximum casualties among our incoming troops," he asserts.
He believes that the Taliban leader Hakimullah Mehsud has moved into dense forests near the border of North Waziristan, where he has cover from US drones.
The army admits Hakimullah is continuing to direct the resistance, and that his men are still on the move, on horses or mules - the traditional transport of South Waziristan.
'Fight to the finish'
Travelling in the area is like going back in time. "Any paved roads you see were built by the British," says one officer, "and that was a long time ago."
Since then there have been decades of neglect, the vacuum left by the state was filled by the militants.
Privately commanders admit that they will have to keep fighting this battle for years to come, if the government does not follow the offensive with a development plan.
For now Pakistanis wonder when the operation will deliver tangible results - like a reduction in the almost daily attacks which claimed around 300 lives in the past month.
They put up a very good fight, [the militants] defended every peak and every ridge, and they filled the area with... improvised explosive devices
General Khalid Rabbani, Pakistan Army
With helicopter gunships circling overhead, the army's chief spokesman - Major General Athar Abbas - was reluctant to give a timescale.
But he maintains that robbing the Taliban of their stronghold will severely limit their ability to strike.
"The people will have to be patient and have confidence that the army will end this as soon as possible," he says.
Previously commanders have said the operation should be concluded in six to eight weeks, though troops would have to remain in the area long after that.
The army has been here before - with three previous offensives since 2004. They ended with peace deals not with victory.
"This time it will be a fight to the finish. We have strong public support," says Maj Gen Abbas.
What Pakistan does or fails to do here has implications beyond its own borders - for security in neighbouring Afghanistan and also in the West.
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