The Pakistani tribal regions of North and South Waziristan have been described as "the most dangerous place on Earth". The recent arrest in the US of Faisal Shahzad for suspected terrorism offences has again put the regions - believed to be the main power base of Taliban and al-Qaeda militants in the country - under the spotlight.
So why is this remote and sparsely populated area so closely associated with militancy?
Many analysts believe the area could harbour some of the world's most wanted men - including al-Qaeda leader Osama Bin Laden.
Such is its reputation that South Waziristan and its surrounding regions have been described by US officials as "the most dangerous place on Earth".
The area was the home of former Taliban chief Baitullah Mehsud, who was killed in a suspected US drone strike in August 2009. It is also home to his successor, the current Pakistani Taliban leader, Hakimullah Mehsud.
North and South Waziristan form a lethal militant belt from where insurgents have launched attacks across north-west Pakistan as well as into parts of eastern Afghanistan.
South Waziristan is considered to be the first significant sanctuary for Islamic militants outside Afghanistan since 9/11. It also has numerous training camps for suicide bombers.
Pakistan's government is under considerable pressure from the US to tackle militancy there.
Analysts also say dislodging al-Qaeda-linked Uzbek and Arab militants in the area is an important goal.
But they warn that will not be easy because North and South Waziristan - in north-west Pakistan bordering Afghanistan - are mountainous regions which provide ideal conditions for militants fighting a guerrilla war.
Both areas are part of the Federally Administered Tribal Areas (Fata), a semi-autonomous region where the central government exercises limited control through a political agent.
Hundreds of families have been affected by fighting in the north-west
For administrative purposes it is divided into two "agencies" - North Waziristan and South Waziristan.
Winters are harsh, making large tracts of the already inhospitable terrain almost inaccessible.
The tribal society found in North and South Waziristan is extremely socially conservative with a fierce reputation as "warriors".
North Waziristan is dominated by the Wazir tribe. This tribe also extends into South Waziristan and makes up one-third of its population. The remaining two-thirds of South Waziristan's population are Mehsuds.
In recent years, Mehsud warriors have traversed the Wazir country to conduct raids against the coalition forces in Afghanistan.
Over a century of skirmishes and warfare in the area have shown why it has been so difficult to subdue.
The British launched several expeditions to quieten different tribes. They were sometimes successful, but often the result was a broken truce or a fatal ambush.
Recent history has not been much different.
Under their chief commander Baitullah Mehsud, the militants twice signed peace deals with the government; first in February 2005, and again in January 2008.
On both occasions, the militants used the respite to regroup and to expand their influence to the rest of the tribal belt as well as parts of North West Frontier Province.
On 12 December 2009, the military claimed victory over the Taliban in South Waziristan and said the militants were now neutralised as a viable threat in Pakistan.
But evidence on the ground suggested otherwise.
The militants' power was amply demonstrated in a wave of suicide bombings and other attacks around the country which left hundreds dead in a matter of weeks. Attacks have continued in 2010 but not at the same rate.
In response to the militant threat to its troops in Afghanistan, the US has recently stepped up drone attacks in the region - especially in North Waziristan.
Drone strikes have killed hundreds of civilians and militants since August 2008.
They have now become a weapon of choice for the US in the fight against the Taliban and al-Qaeda.
Pakistani troops have struggled to root out militant bases
Correspondents say that the White House authorised an expansion of the CIA's drone programme in 2009 following a suicide bombing in Afghanistan which killed seven Americans and a Jordanian at a CIA base.
Over the past two years Pakistan has publicly criticised drone attacks, saying they fuel support for militants, but observers say the authorities privately condone the strikes.
The American military does not routinely confirm drone operations, but analysts say the US is the only force capable of deploying such aircraft in the region.
But while the drones may have succeeded in killing militants in the two Waziristans - in addition to a number of civilians - the harsh mountainous terrain of the area remains a major challenge for the army.
Its gullies, ravines and high mountain trails make arduous going and provide many hiding places for militants.
Knowledge of the terrain is vital - and in this respect the militants have the advantage.
However, the BBC's M Ilyas Khan in Islamabad says that the terrain in the Swat valley, where the army took the battle to the Taliban in 2009, was more difficult than that found in South Waziristan because it is more densely forested.
What might be a deciding factor is the militarised psyche of society in Waziristan. The Wazir and Mehsud tribes are often described by analysts as "born soldiers" willing to fight to the death.
It is a fight which at present shows no sign of ending soon.