Q+A: Why is Pakistan attacking in South Waziristan?

(Reuters) - Pakistani forces exchanged heavy fire with Taliban militants on Sunday, a day after launching a long-awaited offensive aimed at bringing the writ of state to lawless tribal lands on the Afghan border.

The government in June ordered the army to launch an offensive in South Waziristan. Since then the military has been conducting air and artillery strikes to soften up the militants' defenses and blockading the region.

Here are some questions and answers on South Waziristan:

WHY IS THE ARMY ATTACKING?

South Waziristan is the main stronghold of the Tehrik-e-Taliban, or Taliban Movement of Pakistan, an alliance of more than a dozen militant groups that is fighting the government and wants to impose hardline Islamist rule.

The Pakistani Taliban have been responsible for a wave of violence across the country since mid-2007, when the army crushed an Islamist movement linked with South Waziristan based at Islamabad's Red Mosque. As well as numerous suicide bomb attacks against military, government and foreign targets, the al Qaeda-linked Pakistani Taliban were accused of killing former Prime Minister Benazir Bhutto in late 2007.

Taliban leader Baitullah Mehsud was killed by a missile fired by a U.S. drone aircraft on August 5 and replaced by Hakimullah Mehsud who has vowed to exact revenge.

Pakistani Taliban fighters virtually took over control of the Swat valley, northwest of Islamabad, beginning in 2007 and a push out of the valley toward Islamabad spread fear early this year. The army responded with an offensive that has largely cleared militants out of the valley. The army has now set its sights on South Waziristan in the hope of rooting out the most potent domestic threat to the state.

WHO IS THERE?

The army says about 10,000 hardcore fighters are in South Waziristan, an area of about 6,620 square km (2,550 square miles), although estimates vary. Most are members of the region's ethnic Pashtun tribes who have battled intruders for centuries.

Foreign militants, including about 1,000 Uzbeks, some al Qaeda Arabs and even a handful of militants from Western countries are also there. Militant factions from other parts of Pakistan, in particular the south of Punjab province, are also based with the Taliban in South Waziristan.

Osama bin Laden is believed to be hiding somewhere along the Afghan-Pakistani border, though analysts doubt he would be in an area the army is about to attack.

WHAT ARE THE RISKS?

A ground offensive in South Waziristan could be the army's toughest test since the militants turned on the state.

The army launched brief offensives there before, the first in 2004 when it suffered heavy casualties before striking a peace pact. The army has seldom, if ever, ventured into much of the semi-autonomous region of arid mountains and cut by hidden ravines and will be taking on fighters who have had years to prepare defenses. The Pashtun tribes in the region have long been hostile to outside intervention and many people, particularly those belonging to the Mehsud tribe, support the Taliban. So the army could get bogged down in an area in which it has little experience and which is inhabited by a hostile population.

Another risk for the army is that militant factions in North Waziristan might come to the help of their South Waziristan comrades while cells of militants in towns and cities are expected to try to divert attention with attacks like this month's raid on the army's headquarters in Rawalpindi.

(Editing by Alex Richardson)