The Pakistani Taliban, blamed for killing thousands of people in attacks at home, has now grabbed the global spotlight after a failed bid to blow up a car bomb in the heart of New York.
The US administration, initially dismissive of claims by Tehreek-e-Taliban Pakistan that it was behind the Times Square plot, said Sunday it had evidence that the group was involved.
Warlord Baitullah Mehsud founded Tehreek-e-Taliban Pakistan in 2006 as an umbrella organisation of a nebulous collection of militant groups. Four years on, TTP has become the premier national security threat within Pakistan.
Opposed to the 2001 US-led invasion of Afghanistan and supportive of Taliban who fled to Pakistan, the militants were galvanised by Pakistani military operations into their fiefdoms in the semi-autonomous tribal belt after 2002.
Mehsud carved out a headquarters in his native South Waziristan, but members scattered to North Waziristan and Orakzai in the wake of a major army operation in late 2009 and US drone attacks.
Bitterly opposed to the government's alliance with the United States in the war against the Taliban and Al-Qaeda, the group says its aim is to create an Islamic emirate in Pakistan, mirroring Afghan Taliban aims in Afghanistan.
Estimated to have at least 20,000 members, its ranks are drawn from Pashtun tribes in Pakistan's tribal belt. The ethnic group dominates northwest Pakistan and eastern Afghanistan, and Pashtuns also control the Afghan Taliban.
The faction is believed to have ties with the Haqqani network, a powerful opponent of the US military in eastern Afghanistan, Al-Qaeda and militant groups based in Pakistan's southern Punjab and Kashmir.
But the organisation has been described as increasingly fragmented with some commanders reportedly at odds with direct threats on the US homeland. Pakistan insists that the TTP's capabilities have been seriously compromised.
It has been a key architect in a bombing campaign that fanned instability in Pakistan and killed 3,300 people since 2007. Fighting between the military and militants has killed around 2,000 soldiers since 2002.
The government blamed Mehsud for the 2007 assassination of Benazir Bhutto, the first woman prime minister of a Muslim state while she was campaigning for re-election following years in exile.
But the TTP founder was killed in a US missile attack last August, becoming one of the most high profile militants killed in America's covert drone war against Taliban and Al-Qaeda-linked leaders.
Hakimullah Mehsud, a hugely ambitious 31-year-old with a ruthless streak that can frighten his comrades, eventually took over in a leadership struggle, which at one point reportedly degenerated into a shootout.
He vowed revenge for Mehsud's death and the TTP dramatically stepped up attacks in Pakistan, exacting high civilian casualties and drawing comparisons with the nilistic tactics of Al-Qaeda.
TTP's claim of involvement in a suicide attack carried out by a Jordanian killing seven CIA employees in Afghanistan on December 30, marked a departure for a group that had hithero concentrated its efforts in Pakistan.
Hakimullah Mehsud was reported killed in a US missile strike on January 14 in North Waziristan and attacks in Pakistan declined, but he re-appeared in videos allegedly filmed in April, again threatening to attack US cities.
His cousin and the TTP's so-called master trainer of suicide bombers, Qari Hussain, claimed responsibility for the May 1 bomb plot in New York.
Although the claim was initially dismissed, Washington says it now has evidence that the TTP was behind the attack and that the Pakistani-American who allegedly drove the car bomb, Faisal Shahzad, as working at its behest.
Secretary of State Hillary Clinton has warned of "very severe consequences" if an attack against the United States were traced back to Pakistan.