The United States has demanded negotiations. Uganda has threatened to intervene. China has called for an immediate cease-fire. The conflict in South Sudan has attracted attention from around the world, but nowhere are leaders watching the crisis with more interest — and more at stake — than here in the country’s longstanding rival, Sudan. Before South Sudan voted to break away from the north and form its own country in 2011, the two sides had been locked in decades of civil war that claimed the lives of more than two million people. But their division did not sever all ties. The oil that both rely on continues to flow northward from South Sudan’s fields to Sudan’s refineries, linking the two foes in a rocky but crucial economic marriage. The recent fighting in South Sudan has disrupted oil production, with foreign workers fleeing the violent clashes in […]