Economic and trade cooperation has long served as the ballast and propeller of U.S.-China relations. Keeping it steady benefits both countries; moreover, it is a vital global public good. When the world's two largest economies trade and invest with confidence, the benefits ripple outward through global growth, financial markets, and supply chains. When they do not, everyone pays.Personal diplomacy matters when distrust makes contact politically costly. That is why President Xi Jinping's state visit to Washington and his summit with President Donald Trump matter well beyond the headlines. Face-to-face diplomacy between the two heads of state is an irreplaceable strategic anchor. It reduces the risk of miscalculation, builds a measure of trust, broadens cooperation, manages disagreement, and gives officials, professionals, scholars, and companies clear direction for working with their counterparts to solve problems without appearing to concede national interests.A presidential understanding is most valuable when it empowers institutions to do work that personal relationships can only initiate. Such signals lower the political cost of advocating cooperation and raise the cost of gratuitous hostility. And when citizens see their leaders engage in respectful, rational dialogue, it helps defuse toxic nationalism and shows that structural rivals can coexist within an efficacious framework.The business community needs this. According to the U.S.-China Business Council, 95 percent of American companies consider their China operations important to their global competitiveness, and 92 percent were profitable there last year. Yet only about half plan new investment. Profitability, in other words, is not the binding constraint; uncertainty is. Respondents again ranked U.S.-China relations as their greatest challenge.The task now is to convert summit goodwill into predictable, durable institutions, and a start has been made. The two sides agreed on more favorable tariff treatment...