Abiy Ahmed, the prime minister of Ethiopia, was awarded the Nobel Peace Prize on Friday, for his work in restarting peace talks with neighboring Eritrea, ending a long stalemate between the two countries.

Mr. Abiy, 43, broke through two decades of frozen conflict between his vast country, Africa’s second most populous, and Eritrea, its small and isolated neighbor. When he became prime minister of Ethiopia in 2018, he threw himself at a breakneck pace into reforms at home, and peace negotiations with the rebel-turned-dictator Isaias Afwerki, president of Eritrea.

The two nations share deep ethnic and cultural ties, but until July last year they had been locked into a state of neither peace nor war, a conflict that had separated families, complicated geopolitics and cost the lives of more than 80,000 people during two years of border violence.

In its official announcement, the Nobel Committee detailed a litany of accomplishments for Mr. Abiy in his first 100 days as prime minister: lifting the country’s state of emergency, granting amnesty to thousands of political prisoners, discontinuing media censorship, legalizing outlawed opposition groups, dismissing military and civilian leaders suspected of corruption, and increasing the influence of women in political and community life.

“Abiy Ahmed has initiated important reforms that give many citizens hope for a better life and a brighter future,” the Norwegian Nobel Committee said.

But some of Mr. Abiy’s reforms at home, however positive on paper, have also unleashed forces that seem to be beyond his control.