When Janet Yellen left Beijing on Sunday after four days of talks, the US treasury secretary in effect admitted that the delegation achieved its main objective simply by sitting down with top Chinese officials. After years of dangerous and deepening separation between the people running the world’s two biggest economies, they were finally back in a room together. At a brief press conference that was the only tangible outcome of the talks, Yellen listed her hopes for the future. They included something that a decade ago would have been taken for granted: regular “senior-level diplomacy” between Washington and Beijing to...