President Barack Obama has invested more time building personal ties with Chinese President Xi Jinping than with most other world leaders. But as Xi arrived in Washington late Thursday for a grand state visit, it’s clear that Obama’s overtures have produced decidedly mixed results. During intimate walks and hours of private discussion around the world, Obama and Xi forged a historic breakthrough to combat climate change and collaborated on efforts to curb Iran’s nuclear program. But there’s been little progress on a pair of vexing security issues that will be at the forefront of their latest round of discussions: China’s...