We are living through a moment of tremendous transformation. The post-Cold War order is over, and what replaces it is not yet clear. What is clear, however, is that the two countries with the most power to shape that answer are the United States and China. How they manage their competition—in fact, whether they can manage it at all—is a defining question of our era. That question was tested last year, as the two sides skirmished over trade and technology. It will be tested ag