Ethiopia’s Nobel Peace Prize-winning prime minister on Wednesday ordered the military to confront the country’s Tigray regional government after he accused it of a deadly attack on a military base, declaring “the last red line has been crossed” after months of alleged provocations. Prime Minister Abiy Ahmed’s move against the well-armed Tigray People’s Liberation Front raised concerns that one of Africa’s most populous and powerful countries could plunge into civil war. That would send a shock wave through one of the world’s most turbulent regions, the Horn of Africa. Ethiopia’s neighbors include Somalia and Sudan, and the prospect of spreading...