The assassination of Mr. Burhanuddin Rabbani is a body blow to the Afghanistan peace process. The killing Sept. 20 demonstrated once again the ability of the enemies of peace in that country to penetrate the inner reaches of government as well as the seemingly implacable opposition of the Taliban to real peace.

The killing is another sign of the possible difficulties awaiting Afghanistan when coalition forces complete their withdrawal from the country two years from now.

Mr. Rabbani, who served as president of Afghanistan from 1992-96, was the leader of Afghanistan’s High Peace Council, a 70-member body that included all members of the country’s political continuum. It had been set up almost a year ago to try to reach a peace agreement with the various insurgent groups fighting the government in Kabul.

Mr. Rabbani, an ethnic Tajik and a member of the anti-Taliban Northern Alliance, made his reputation as one of the founders of the mujahedeen movement that fought the Soviets in the 1980s. In Afghanistan’s deeply divided society, he was considered a moderating influence who could reach across tribal lines. Few political figures could match his stature.

Mr. Rabbani was killed by an assassin who had hidden explosives in his turban. The meeting was part of an ongoing effort to reach out to disaffected members of the Taliban and to try to bring them into the government.

The killer had reportedly been in contact with the High Peace Council for five months and had requested a meeting to pass on “a serious … very important and positive message” from the Taliban leadership group.

Upon arriving in Mr. Rabbani’s home, the two men hugged and the bomb went off, killing Mr. Rabbani and the assassin and wounding several others. The Taliban claimed responsibility for the assassination and warned of more attacks to come.

The direct impact on the peace process is uncertain. That is because the High Peace Council has had only limited success to date; some blame Mr. Rabbani, who was seen as a well-meaning but somewhat ineffectual figure.

While he had crossed the country setting up local peace councils and traveling outside Afghanistan to meet rebel leaders, there was little evidence of real progress in the peace process. Few guerrillas had laid down their weapons and joined the High Peace Council.

Some observers claim that real peace talks were in fact being held by Americans with the Taliban and were largely independent of Kabul’s efforts. Those talks have been proceeding, but their progress is also uncertain.

There have been embarrassments when representatives claiming to be from the Taliban were exposed as not being who they said they were. Yet, even if this track moves forward, ultimately, the Afghan government has to be a party to negotiations if the outcomes are to be credible and endure beyond the 2014 pullout of coalition forces.

The question is whether the murder is a definitive rejection of all peace talks by the Taliban or “just” the settling of scores within Afghanistan. The Taliban are divided and it is not clear whether there is any single power center that can speak for the entire movement.

According to this interpretation, the killing reflects a power struggle within the Taliban. At the same time, the murder is fracturing the government and its allies.

Immediately after the assassination, several Northern Alliance leaders denounced the Taliban and the peace talks, insisting that the militants were not sincere.

A more compelling explanation suggests that the Taliban senses weakness in the Kabul government and is doing its best to make the government of President Hamid Karzai appear weak, temporary and illegitmate before the 2014 handover.

This killing comes on the heels of other equally brazen attacks on key figures, including Mr. Karzai’s brother, who was killed in Kandahar in his home in July, and the murder of the top northern police commander Gen. Dawood Dawood.

The week before last, militants penetrated deep into the heart of the most heavily guarded area of Kabul to attack the U.S. embassy and the coalition forces headquarters, another sign of the Taliban’s ability to strike at will.

Mr. Karzai, who was in the United States attending the United Nations General Assembly when the killing occurred, cut short his trip and returned home, vowing to continue efforts to reach out to the Taliban.

Mr. Karzai is right: The talks should continue. The depth of Afghanistan’s ethnic and tribal divisions demands a coalition government that is as broad as possible.

While the aim is inclusivity, an enduring agreement will depend on the isolation of factions within the broader Taliban movement that are completely opposed to peace. Those groups must be identified, drained of resources and deprived of the ability to wage war. Their supporters must also be identified and given stark choices about the roles they seek to play in Afghanistan’s future.

All parties must also adopt a sense of urgency. The withdrawal date for coalition forces is less than 2½ years away. Failure to move forward will force other parties to begin preparing for the worst case scenario — yet another civil war.

That does not have to be Afghanistan’s future, but it looks increasingly likely if supporters of peace do not double down their efforts.

In a time of both misinformation and too much information, quality journalism is more crucial than ever.
By subscribing, you can help us get the story right.