One American view of China — increasingly popular on the left and among the hate-Trump crowd on the right — is that the communist colossus will be forever ascendant, boasting astonishing levels of food production, ship construction and industrial output. In this pessimistic view, China will soon replace America as the world’s predominant power. Yet even Beijing’s miraculous 30-year leap out of poverty into first-world affluence is hardly the same as parity with the United States. In truth, President Donald Trump held almost all the cards at last week’s Beijing summit — and will do so again when Xi Jinping...