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When does a rising power become a threat? There is seldom a single moment. A century ago, AngloGerman antagonism was still a relatively new phenomenon; an alliance between the two empires seemed plausible as late as 1899. Likewise, the United States took time to identify Japan as a serious rival in the Pacific region; it was not until the 1930s that relations really soured. In both cases, the perception of a strategic threat was slow to grow. But grow it did"”and ultimately it led to war. Could the same be happening to the United States and China today? Are we imperceptibly but inexorably slipping from cooperation to competition?

Back in early 2007, it seemed as if China and America were so intertwined they'd become one economy: I called it "Chimerica." The...