The convoy rumbled into the Taliban heartland, a white desert littered with stones. Over the loudspeakers at the local mosque, the Afghan police officers ordered everyone to gather: The commander was here.

Dozens assembled in the mud square to listen as Abdul Raziq, one of America’s most important partners in the war against the Taliban, stood before the crowd, gesturing at two prisoners he had brought along to make his point.

The prisoners knelt with their hands bound as Raziq spoke to his men. A pair of his officers raised their rifles and opened fire, sending the prisoners into spasms on the reddening earth. In the silence that followed, Raziq addressed the crowd, three witnesses said.

“You will learn to respect me and reject the Taliban,” Raziq said after the killings, which took place in the winter of 2010, according to the witnesses and relatives of both men. “Because I will come back and do this again and again, and no one is going to stop me.”

For years, American military leaders lionized Raziq as a model partner in Afghanistan, their “if only” ally in the battle against the Taliban: If only everyone fought like Raziq, we might actually win this war, American commanders often said.