Ethiopia’s military marched into the main city of the northern region of Tigray on Sunday after the authorities there retreated.

That withdrawal from the regional capital, Mekelle, is a victory for Prime Minister Abiy Ahmed, less than two weeks after fighting between the federal government and the authorities there escalated sharply.

The two sides fought a two-year war beginning in 2020 that claimed hundreds of thousands of lives, with reports of widespread atrocities and mass sexual violence. The new hostilities in Ethiopia, Africa’s second most populous country, have caused global alarm, with fears that they could spiral into a regional conflict involving other countries in the Horn of Africa and the Gulf.

Who is fighting?

The conflict pits the Ethiopian National Defense Force, the army of the federal government based in the capital, Addis Ababa, against the Tigray People’s Liberation Front in the north.

Last month, the T.P.L.F. joined six armed opposition groups to form a new coalition, the Alliance for Survival. Their aim is to topple Mr. Abiy, who has been in power since 2018, and to reform the Constitution.