Militant: Pakistani Taliban fighters training in South Waziristan tribal region in 2011.

The Pakistani Taliban, formally known as Tehrik-i-Taliban Pakistan, is a loose and increasingly divided umbrella organisation that once represented about30 groups of militants. The group was officially founded in 2007 by a prominent jihadi commander, Baitullah Mehsud, and for years it and allied groups like al-Qaeda have been based in the Pashtun tribal areas in north-western Pakistan, particularly in North and South Waziristan.

Many Pakistani Taliban commanders fought in Afghanistan as part of the movement that swept to power in Kabul. When US forces ousted that movement in 2001, many of its leaders fled across the border into Pakistan. The Pakistanis among them played host to their Afghan counterparts - as well as hundreds of fighters from al-Qaeda - providing them with shelter, logistical support and recruits.

Under pressure from the United States, the Pakistani army made tentative efforts to dismantle those sanctuaries in 2003 and 2004, but it was too late. The tribal militiamen, enriched and radicalised by their al-Qaeda guests, chafed under the army's attempts to impose control. The United States designated the Pakistani Taliban a terrorist organisation in September 2010.

Protected by jihadists: Al-Qaeda chief Ayman al-Zawahiri.

The group owes allegiance to the Afghan Taliban leader, Mullah Mohammed Omar, and co-operates closely with the Afghan movement in its insurgency in Afghanistan, providing men, logistics and rear bases for the Afghan Taliban. It has trained and dispatched hundreds of suicide bombers from Pakistan's tribal areas.

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The movement shares a close relationship with the Haqqani Network, the most hard-core affiliate of the Afghan Taliban, which has been behind repeated suicide attacks in and around Kabul and eastern Afghanistan. The groups also co-operate and provide haven for al-Qaeda operatives, including al-Qaeda's leader, Ayman al-Zawahiri.

The wide extent of militant co-operation in the tribal areas has complicated matters for the Pakistani military intelligence agency, which has long provided support for the Afghan-focused Taliban, even while trying to fight the Pakistani Taliban in recent years.

One of the Pakistani Taliban's most significant attacks in 2014 was an audacious siege of the Karachi international airport in June. The attack, in which a group of 10 attackers fought security forces for hours and killed 13 people, represented the final straw for Pakistan's military. Within days, an extensive military air and ground assault began against Taliban leaders headquartered in North Waziristan. It is that offensive that a Taliban spokesman said led to the retaliatory militant attack in Peshawar on Tuesday that killed dozens of Pakistani schoolchildren and teachers.

In September 2013, the Pakistani Taliban unleashed one of its deadliest attacks ever, sending suicide bombers to the historic All Saints Church in Peshawar, a symbol of co-operation between Muslims and Christians. All told, at least 120 people died in the attack and its aftermath, which refocused attention on the Taliban's persecution of religious minorities.

In 2012, the Pakistani Taliban shot Malala Yousafzai, a Pakistani schoolgirl in the Swat Valley, for advocating the education of girls. Malala went on to win the Nobel Peace Prize in 2014 and has become a worldwide symbol of the group's indiscriminate violence and subjugation of women and girls.

Mehsud is also thought to have been behind the suicide bombing that killed former Prime Minister Benazir Bhutto in December 2007.

Under Hakimullah Mehsud, who took over command of the Pakistani Taliban after the death of Baitullah Mehsud, the group demonstrated a close alliance with al-Qaeda. 

The Pakistani Taliban group is now nominally led by Maulana Fazlullah, a jihadi leader thought to be in hiding on the Afghan side of the border.

Mr Fazlullah was seen as a possible peacemaker within Pakistan's militant firmament when he was chosen to lead the Taliban after the previous leader, Hakimullah Mehsud, was killed in a US air strike in November 2013.