Throughout a decade of terrible conflict in their country, there is one kind of violence Afghans have largely avoided: between Sunni and Shia. Despite the sectarian tensions that have splintered much of the Muslim world since September 11, there were no major sectarian attacks in Afghanistan between 2001 and the fall of 2011. To the contrary, the Taliban, who adhere to the conservative Deobandi sect of Sunni Islam, have taken extra care not to aggravate Afghan Shia, who make up an estimated 10 to 15 percent of the population.

On December 6, however, all that changed. In coordinated attacks aimed at Shiite Muslims in three Afghan cities, bombs killed 63 people and wounded 150. Lashkar-e-Jhangyi, a Pakistani militant group affiliated with al-Qaeda, has claimed responsibility for the attacks. Its aim appears to be to start a sectarian civil war. This is a tactic al-Qaeda has used in Iraq, Pakistan, and most recently Egypt, and indicates a dangerous change in direction in the Afghan conflict—even as US and its Western allies prepare to leave the country. (The bombs came a day after the conclusion of an international conference in Bonn where the US and NATO pledged to continue helping Afghanistan after troops withdraw in 2014. )

The worst attack took place in Kabul, where a suicide bomber walked into a crowd of religious mourners and detonated his bomb, killing at least 59 people. A similar walking bomber detonated himself in the northern city of Mazar-e-Sharif, killing 4 and wounding twenty, while a third bomb left on a bicycle in a bazaar in Kandahar missed the crowd and wounded two policemen. The attacks took place on the tenth day of the Muslim month of Muharram, the end of Ashura, the holiest ten days in the Shia calendar when all Muslims but particularly Shias commemorate the death of Imam Hussain, the grandson of the Prophet Mohammed.

Inevitably, suspicions that Pakistan may have had a hand have arisen in Afghanistan. However, the fact that a Pakistani group claimed responsibility for the attacks does not, in itself, point to the involvement of the Pakistani state. For more than a decade, Lashkar-e-Jhangyi (LJ)—a militant Sunni organization that was founded in 1996 as a splinter group of Sipah Sahaba, the principle anti-Shia party in Pakistan—has been working to upend the Pakistani state itself. Indeed, the Sipah Sahaba was banned in Pakistan as a terrorist organization in 2002 and Lashkar-e-Jhangyi is a virtual arm of al-Qaeda.

Pakistan must now help the Afghans investigate the involvement of Lashkar-e-Jhangvi, and crack down hard on the group in Pakistan, if it hopes to allay suspicions in Kabul. It will have to act quickly and transparently. And since Lashkar-e-Jhangyi represents a threat to Pakistan as well as to Afghanistan and to the Americans, this should be an opportunity for all three nations to improve their battered relationships by working together to crush such groups.

Afghan Shia are largely made up of the country’s Hazaras – an ethnic group descended from Genghis Khan and the Mongols who are concentrated in the Hazarajat, the highlands region of central Afghanistan. Between 1992 and 2001 the Hazaras were repeated victims of sectarian massacres, first by Tajiks and Uzbeks who fought each other in the bloody civil war and then at the hands of the Pashtun Taliban, who set out to conquer the country in 1994 and carried with them an ideological hatred for Shia as non-believers.

However the worst massacres of Hazaras took place after 1996 when Taliban commanders were joined by Arab fighters loyal to Osama bin Laden and Pakistani fighters belonging to Lashkar-e-Jhangyi and other groups, who deliberately sought out Shia Hazaras to kill. (Riaz Basra, the founder of Lashkar-e-Jhangyi, believed that the party from which it split had abandoned its main mission, which was to kill Shia Muslims). These fighters had trained and fought in Afghanistan, first with the Taliban and then with al-Qaeda, while also mounting attacks in Pakistan aimed at prominent Pakistani Shia, many of whom were killed.

Initially Lashkar-e-Jhangyi was close to the Pakistan military and its Interservices Intelligence, which also supported the Taliban. But even before 2001 LJ militants had turned against the government. Since 2001 they have bombed military targets in Pakistani cities, helped al-Qaeda hide out in Pakistan and are now part of the Pakistani Taliban movement, which aims to overthrow the Pakistani state. According to Human Rights Watch, since 2008, over 275 Hazaras have been killed in Pakistan’s Balochistan province, which borders Afghanistan. In two attacks in Balochistan claimed by LJ earlier this fall, 39 Hazara Shia were shot dead.

By contrast, in Afghanistan, the Taliban have deliberately avoided the Hazarajat and have not mounted attacks against Hazaras since they began their insurgency against US forces in 2003. The Afghan Taliban have tried to show that they are no longer controlled by Arab or foreign sectarian fanatics, that their “jihad” would unite all Afghans against the Americans, and that, as Pashtuns, they were not against other ethnic groups. Just last month Taliban leader Mullah Mohammed Omar warned his fighters not to target civilians.

Moreover, the Hazaras and their powerful military commanders have been supported by Iran, which has continued to fund their political and religious activities. As part of its anti-American policy, Iran also backs several Taliban groups that operate in western Afghanistan and has used its influence to warn the Taliban not to touch the country’s Shia minority.

But there is now a growing rift between those Taliban who want a political solution before the US and NATO leaves, and more hard-line groups. Al-Qaeda and some Pakistani extremists are keen to thwart the secret talks being held between the Americans and the Taliban, undermine the Afghan government, and prevent a longer-term US presence to continue in Afghanistan after 2014 when most Western forces will leave. And part of al-Qaeda’s way of accomplishing this appears to be to revert to the now familiar strategy of starting a sectarian bloodbath.

The Sunni-Shiite war that raged in Iraq from 2004 to 2007 was the main tool used by the group that called itself al-Qaeda in Mesopotamia to try and sow chaos in the country, defeat the American occupation, and undermine Iranian influence there. Pakistan, where Shia comprise some 15 to 20 percent of the population has suffered from sectarian war since the early 1980s. But sectarian killings escalated dramatically after al-Qaeda began to recruit Pakistani Sunni extremist groups to its side in the 1990s. Al-Qaeda is now trying to do the same in Egypt. In order to undermine the revolution and the public clamor for democracy, al-Qaeda has been calling for attacks against the Coptic Christian minority whose churches have been burning, seeking to create a Christian-Muslim religious war that could supplant Egypt’s struggle for democracy.

In Afghanistan the fact that sectarian killings subsided so rapidly after 2001 when the Taliban were defeated, demonstrated that sectarianism is not deeply rooted. And the prospect of a general sectarian civil war seems unlikely, in view of the relative absence of such conflict in Afghan history, apart from the 1990s. But the re-introduction of sectarian killing by non-Afghan groups has dangerously widened the scope of the war in Afghanistan and threatens to draw in neighboring countries such as Iran and Pakistan. And it is one more way to keep Afghanistan permanently in a state of conflict.

Update: the original version of this post gave an estimate of 3 million for the Afghan Hazara population, following published sources. In fact, there has been no census for several decades and the exact number is unknown. The text has been corrected.