Some of Africa’s most entrenched leaders are facing an unprecedented challenge from an unexpected foe: falling oil prices. Rulers of a cluster of nations along Africa’s oil-rich Atlantic coast have long used crude revenue to consolidate their power, reward political allies and subsidize basic goods and services. As oil prices decline, that formula for maintaining power looks increasingly inadequate for rulers such as Angola’s José Eduardo dos Santos. “If the drop in prices continues, he will be in free fall,” said Ricardo Soares de Oliveira, a political scientist at the University of Oxford. For more than 30 years in office, Mr. dos Santos has relied on the proceeds from oil sales. If that revenue continues to shrink, “the very assumption that this regime is able to weather all sorts of […]