Oil dispute challenges China’s long-standing policy of not interfering in other countries’ internal affairs. On the day South Sudan became independent last year, China opened an embassy here, eager to protect its oil interests. It quickly dispatched its foreign minister and began discussing a huge aid package for this destitute land. Just a few months later, Beijing finds itself trapped in a bitter wrangle between South Sudan and its former rulers in Sudan, with both countries pressing Beijing to take their side...