* Pakistan launches long-expected military operation
* Concerns that militants are slipping away across border
* Pakistan under U.S. pressure to crush militant hideouts
After months of dithering, Pakistan's army has launched an offensive against Taliban insurgents near the border with Afghanistan but the tough terrain, a potentially hostile local population and the possibility of revenge attacks in heartland cities could be more difficult to conquer than the militants.
Pakistan announced on Sunday it was sending ground forces, artillery and helicopter gunships to the remote, mountainous tribal region of North Waziristan in a long-awaited military operation designed to eliminate the al Qaeda-linked insurgents.
Islamabad has been under intense U.S. pressure for years to crush sanctuaries for militants in the region and Pakistan's move will be greeted with resolute approval in Washington - but the challenges facing its army on the ground mean it should be ready for a long haul.
No single outside force has ever succeeded in subduing the volatile ethnic Pashtun region straddling Pakistan's western frontier with Afghanistan, its deeply tribal population fiercely independent and opposed to any invading army.
The biggest setback may be far away from the battlefields of North Waziristan as the country braces for a wave of Taliban revenge attacks around Pakistan including in Punjab, Prime Minister Nawaz Sharif's political power base.
"The biggest challenge of this operation is that our success in the tribal areas could quickly turn into losses in the plains of Punjab," a senior military official close to the operation told Reuters.
"Because there will be blowback and the public will get scared and Taliban sympathisers will come out and say 'we told you so'. And that's where we could lose this battle."
"That is Nawaz Sharif's biggest challenge," he added: "To convince the public that it's better to bleed once than to die slowly everyday."
Pakistan, a Muslim nation of 180 million, has tacitly supported several Islamic militant groups in the past, especially those opposed to the current government in Afghanistan and those fighting India's rule in disputed Kashmir.
But the Pakistani Taliban, a loose alliance of radical groups, is fighting to overthrow the Islamabad government and impose a strict form of Islamic law on Pakistan.
The government was attempting to hold talks with moderate Taliban leaders but those broke down after a brazen command-style attack on Karachi airport this month in which a group of highly trained fighters tried to run over the facility and hijack a passenger plane.
In response, Pakistan has doubled its ground forces in North Waziristan to about 80,000 troops, military sources said, in preparation for a major ground operation against insurgent strongholds.
GROUND OFFENSIVE SOON
So far the army has resorted only to air strikes, sending F-16 and Mirage fighter jets to pound suspected militant hideouts up in the mountains in a strategy to disorient the Taliban and sow panic among their ranks.
According to a military official close to the operation, the ground offensive will start in the next week when land forces will try to comb through North Waziristan's valleys and take over villages and buildings.
Air raids have continued daily since Sunday, killing hundreds of fighters and no civilians, according to Pakistani military sources. The official account is impossible to verify as journalists are not allowed to work freely in the region.
The success of any operation of this scale is impossible without the involvement of the United States, whose forces in neighbouring Afghanistan have crucial intelligence on the location of militant bases and training camps around the region.
Pakistan fears the militants may slip over the border into Afghanistan once the offensive starts and indeed some senior leaders may already have. Pakistan says it has asked Afghanistan's army to help seal off the border from its side.
But with most U.S. troops leaving Afghanistan this year, it is unclear how much capacity and willingness Washington would have to get involved in another conflict far away from its shores.
Once the ground operation gets under way, analysts also expect the notorious lack of coordination among Pakistan's myriad of security and intelligence agencies to hamper efforts to tackle the insurgency head-on.
"The remote and rugged terrain is a big problem but the biggest challenge is away from the tribal areas," said a close aide to the prime minister. "It's a question of intelligence coordination throughout the country.
The Taliban are deeply entrenched in Waziristan's complicated patchwork of tribal alliances, blending into the local population and making it hard to distinguish them from ordinary residents.
"The biggest challenge will be intelligence, how to get precise intelligence and then go after them," said Imtiaz Gul, a security analyst. "They are dealing with a mobile enemy. It pops up here and there, and wherever you apply pressure they move to another place."
Pakistan's strategy, for now, is to encircle North Waziristan with troops and use helicopters and fighter jets to attack sanctuaries from the air.
Before it launches a ground offensive, the army has given the region's estimated two million population several days to evacuate the area, with a large number of refugees massing in a tent camp across the border in Afghanistan's Khost province - a potential humanitarian crisis in the making.
Those who have stayed behind are unlikely to give troops a warm welcome, analysts say, particularly in areas with traditionally strong Taliban influence.
Even if the army's advance through the region is smooth, it is unclear what would happen afterwards and how Pakistan intends to rebuild the ruined villages to bring the refugees back.
A similar operation in South Waziristan in 2009, which was unpopular among Pakistanis, displaced half a million people as homes, schools and hospitals were turned into hideouts by militants and meagre civic amenities were destroyed. The region remains largely undeveloped.
"The most difficult task is not the operation, they can achieve that and clear the area," said Muhammad Amir Rana, director of the think tank Pak Institute for Peace Studies.
"It is in the post-operation period when many different scenarios emerge. (Taliban) support networks, their affiliates are still active in many different parts of the country."
So far air strikes have been targetting mainly Uzbek strongholds in North Waziristan. Allied with the Pakistani Taliban, they have no tribal affiliations in Pakistan and are seen mainly as al Qaeda's foot soldiers with little clout.
Pakistan has always distinguished between the good and the bad Taliban, identifying some as moderates with whom the state can negotiate but the breakdown of talks has changed the picture.
"It's difficult to distinguish between the good and the bad Taliban. It wouldn't be an ideal scenario if the good and the bad Taliban joined forces and attacked the army together," said Saifullah Mahsud, head of the FATA Research Centre think tank.
"That would make things very difficult for them."