US president Joe Biden’s upcoming meeting with Chinese president Xi Jinping is expected to be the most important and consequential bilateral encounter this year. The South China Sea and Taiwan threats, meanwhile, continue to hover over the summit. The course of events in the year since the two leaders’ previous meeting gives cautious hope that they will have greater luck this time. But it also shows how easily any agreement they reach could once again veer off course. The US and Chinese presidents will meet in San Francisco on Wednesday as part of the annual Asia-Pacific Economic Cooperation summit. They...