This year’s Chinese National Day reception in Washington felt noticeably jollier than those I have attended in recent years. The mood was more relaxed and Chinese Ambassador Xie Feng’s opening speech was upbeat. He even joked about the panda ambassadors at his own expense. The cheerful tone followed President Xi Jinping’s short but successful state visit to the US, during which his counterpart, President Donald Trump, gave him an unprecedented high-level reception that would have flattered even...