Despite its unchallenged military might, the United States has an Achilles' heel: its economy depends on foreign capital. Though hardly anyone acknowledges this publicly, China and Japan already hold so much American debt that, theoretically, each could exert enormous leverage on American foreign policy. So far, the economic dependence of these countries on American consumers has kept them from exercising such power. But what would happen if, for instance, Washington changed its one-China policy and officially recognized Taiwan?