By M Ilyas Khan
BBC News, Islamabad
Hundreds of families are abandoning their homes to escape the fighting
South Waziristan is spread over an area of more than 2,700 square miles, two-thirds of which is inhabited by the Mehsud tribe.
The Mehsud country is hemmed in on the north and west by the Wazir tribe, and on the east by the Bhittani tribe of Tank district.
In the south, they have long been in dispute with their Wazir cousins over the ownership of the Gomal pass.
As such, the Mehsud have no direct access either to Afghanistan or to the Pakistani settled areas.
This geographical isolation of the Mehsuds, and the barren hills of the north-eastern parts of South Waziristan which they occupy, have made the Mehsud country largely unattractive to rulers to the east and west.
But the inherent militancy of the Mehsud psyche has often invited punitive expeditions into their area, from the British colonial period to the present.
During the days of the British, they struck temporary alliances with the Bhittani tribesmen and used their area to launch raids in British territories to the east.
In recent years, Mehsud warriors have traversed the Wazir country to conduct raids against the coalition forces in Afghanistan.
The British launched several expeditions to subdue the tribe. They were sometimes successful, but often the result was a broken truce or a fatal ambush.
The recent history is not 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 the settled districts of the North West Frontier Province.
The Mehsud militancy stems from their lifestyle and their resources, or the lack of them.
Like most people across the Pakistani tribal borderland, the Mehsuds live in vast, fortified mud compounds built to defend against the enemy.
For these people, no purpose in life is higher than to avenge a death in the family, in the clan or in the tribe.
Baitullah Mehsud led the charge on behalf of the militants
Among these people, the standing of the Mehsuds as the most ferocious fighters is linked to their geographical and demographic position vis-a-vis their tribal cousins, the Wazirs.
While the Mehsuds constitute two-thirds of the population of South Waziristan, they are just a small offshoot of the Wazir tribe in the wider Waziristan region.
In addition, while the Wazirs control some of the most fertile valleys and lucrative trade routes on the border, the Mehsuds are left with largely barren mountains and barely an exit route which they can undisputedly call their own.
So, a suppressed anger towards their better-off cousins, and the compulsion of having to rob others to provide for themselves appear to be the two most basic socio-economic sources of their militancy.
This is further helped by their terrain, which at first sight appears to be a tangled mass of mountains and hills running irregularly in all directions, but is actually hemmed in by well defined ranges on the outer periphery that make penetration of the area difficult.
The British, with their 19th Century equipment and the reputation of being an occupying force, often failed to beat them conclusively on their own turf.
The Pakistanis, endowed with 21st Century technology, almost subdued them in January 2008, and think they can finish the job this time around.
Apart from modern technology, the Pakistani forces have one more advantage.
The Taliban and al-Qaeda militants that hold the Mehsud country today are not the equivalent of the tribe.
In many ways, they have challenged the ways of the tribe, have used intimidation to galvanise public support, and have generally divided society.
As such, their mission does not conjur up sentiments of tribal pride that the Mehsuds have so ferociously defended in the past.