As Obama concluded his visit to China today, many commentators paused to comment on the long-foreseen changing relationship between the United States, superpower of the twentieth century, and its developing Asian rival. The BBC's Matt Frei provides some much-needed grounding for the common intuition, and puts the change in concrete terms. The power shift is undoubtedly tied to China's role as America's banker, he says, but that is not the whole story. America, he writes "is now more in awe of China than vice versa." He provides examples of American vulnerability from recent years: images of U.S. troops stymied by desert fighters armed with homemade bombs, and of American democracy hostage to "stonewalling, sandbagging and petty in-fighting on Capitol Hill." Meanwhile, China has "established closer ties with countries like Brazil and Peru in what used to be called America's backyard."

Bringing together a number of themes that have been flitting around the opinion world in past weeks, he avoids any one reductionist explanation. He includes material causes, but also discusses China's new equivalent of the American Protestant ethic--"boundless optimism" married to patriotism and patience. (David Brooks devoted a much-cited column to it this week.) Yet Frei also gives weight to the proliferation of Chinese "factories and engineers ... all over Africa with the same mercantilist zeal once displayed by the East India Company."

When all is said and done, Frei neither declares China's ascendancy nor America's decline a fait accompli. The United States is still a country of great ideals, not least freedom of the press. Nor is China free of its own problems: "The moment of greatest danger," he writes, "will come when China's Internet savvy, iPhone-wielding, BMW-driving, condo-owning, well travelled middle class wants to be treated like adults and the regime continues to talk down to them like children." The picture Frei paints, therefore, is pointed but complex--a snapshot of shifting empires.