By DAVID E. SANGER and RICK GLADSTONE AUG. 8, 2016 NYtimes

When President Xi Jinping of China visited President Obama at the White House last September, he startled many with reassuring words about his intentions for the Spratly Islands, a contested area where the Chinese government has been piling dredged sand and concrete atop reefs for the past few years and building housing and runways on them.