Mullah Baradar, one of the Taliban’s founding leaders, was instrumental in their regrouping as a strong insurgency after the United States toppled their regime in 2001. Around eight years later, he made contacts with the Afghan government at the time, drawing the ire of the Pakistani military, which has been supporting the Taliban as a proxy force and giving its leadership sanctuary.

The mullah was arrested in a joint operation of Pakistani and American agents, at a time when the United States military was still pursuing a victory on the battlefield and had not embraced a political resolution to the war.

Mullah Baradar was released from Pakistani custody in October at the request of the American government, which hoped he could help end a war the Trump administration desperately wants behind it. But his delay in arriving in Doha repeatedly raised doubts about how free he actually was — with Afghan officials and some Taliban sources suggesting Pakistan was still closely watching him.

Mullah Baradar’s arrival for peace talks gives the Americans a senior interlocutor who can finalize a deal and then use his weight with the Taliban’s rank and file to put it in place. Even so, moving the peace talks to the next stage, with Afghan government involvement, still seems daunting.

The Taliban have refused to meet with the Afghan government, while President Ashraf Ghani has insisted that the peace talks will not go anywhere unless the insurgents formally sit with his administration.

A large group of Afghan politicians, led by former President Hamid Karzai, met with the Taliban in Moscow this month in what they hoped could be an icebreaker for broader talks between the groups. But officials in the Ghani government heavily criticized the move, saying it undermined the fragile Afghan state at a critical time.

The government and the opposition have also vehemently disagreed over the makeup of a negotiating team for when the Taliban do agree to formal talks with Afghan officials. Mr. Ghani’s choice — a group of negotiators that would include several government ministers and be led by his chief of staff — was denounced as unrepresentative by the opposition. Mr. Khalilzad, the American envoy, has also said the government would have to make its team inclusive for negotiations to move forward.