The U.S. diplomat arrived in West Africa in mid-June with little fanfare, hopscotching along the Atlantic coastline for scheduled visits with senior government officials in the Ivory Coast, Gambia, Guinea-Bissau, Togo and Gabon, whose president greeted him inside an opulent seaside palace. The American, a career State Department official named Christian Ehrhardt, was an unlikely diplomatic VIP. For nearly two decades, he served as an officer in the agency’s security branch, focusing on embassy protection, before being summoned to Washington last year to lead a new initiative — the Office of Remigration. Since then, Ehrhardt, 41, has crisscrossed Africa, trying...