KABUL, Afghanistan — An assassin with explosives hidden in his turban was ushered into the home of the head of Afghanistan’s peace process on Tuesday, embraced him and then exploded the bomb, killing him and dealing a potentially devastating blow to the effort to reconcile with the Taliban and end 10 years of war.

The assassination of Burhanuddin Rabbani, the leader of Afghanistan’s High Peace Council and a former president, on the heels of a carefully planned attack on the American Embassy a week ago, underscored the fierce opposition of those who want to shatter the country’s tenuous stability and thwart its tentative steps toward peace.

It also demonstrated once again the ability of the government’s enemies to reach into even the most secure bastions of the capital, whether through treachery or frontal assaults, and to carry out a rising number of carefully selected assassinations. This one, however, may be the most significant of the war.

Without the 71-year-old Mr. Rabbani, it will be exceedingly difficult to move the peace process forward. A complex figure, he was nonetheless one of the few with the stature to persuade the Taliban’s enemies, the former Northern Alliance, to embark on reconciliation discussions.