Ethiopia’s prime minister, Abiy Ahmed, was awarded the Nobel Peace Prize on Friday for his work toward ending a 20-year conflict with neighboring Eritrea and opening up one of Africa’s most notorious one-party systems.

Mr. Ahmed’s overhauls have put Ethiopia, Africa’s second-most-populous country and one of its fastest-growing economies, on a transition toward democracy, even as other countries on the continent have drifted deeper into authoritarianism.

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