On paper, the pitch was simple: A green energy company backed by $217 million in U.S. government loans would convert one of Africa’s poorest countries into the world’s first biomass-driven economy. But the plan to help Liberia collapsed amid questionable business decisions and oversight. The company, Buchanan Renewables, dismissed 600 workers and left the country amid complaints of workplace injuries, environmental harm and, at times, sexual abuse. Backing the company at every stage was the Overseas Private Investment Corporation, a federal agency with a global mandate but low profile. The agency approves more than $3 billion a year in financing,...