It’s not the first time the Nobel committee awarded the peace prize to fortify a new leader, early in his term, whose lasting achievements were still uncertain. In 2009, the prize was given to President Barack Obama, only nine months into his term, with the committee citing his advocacy of nuclear disarmament and “his extraordinary efforts to strengthen international diplomacy and cooperation between peoples.”

When Mr. Obama’s disarmament efforts stumbled, the Nobel committee came under increasing criticism for awarding the prize based on aspirations, not achievements.

Mr. Abiy’s record, too, is still being written. He has attempted to extend his high-energy conflict resolution efforts beyond his country, to the region. He personally intervened this year in neighboring Sudan to compel military and civilian leaders there to sign a power-sharing agreement — a settlement that is holding so far but could prove shaky.

The peace accord signed more than a year ago between Mr. Abiy and Mr. Isaias, in Eritrea, has only slowly translated into concrete steps to reconnect the two nations. Genuine change for Eritreans has been limited: They remain isolated and under Mr. Isaias’s iron fist despite the peace.

But the agreement has been held up as an example of how historic change can come about in even the oldest and most intractable conflicts. Diplomatic relations between Ethiopia and Eritrea have resumed, and the two leaders and senior officials from both nations have met frequently to discuss how to reconnect the two countries.

“Peace does not arise from the actions of one party alone,” Ms. Reiss-Andersen said as she announced the award on Friday. “When Prime Minister Abiy reached out his hand, President Afwerki grasped it, and helped to formalize the peace process between the two countries.”

Telecommunications between Ethiopia and Eritrea have been restored, allowing families that were split in the war to regain contact. In the days that followed this breakthrough, some Ethiopians called Eritrean numbers randomly, and vice versa, just to speak to someone on the other side, simply because they could. Others tracked down parents, siblings and friends.