Japan and South Korea are Western-style democracies with open-market economies committed to the rule of law. They are also U.S. allies. Yet despite their shared interests, shared values, and geographic proximity, divergent national identities have driven a wedge between them. Drawing on decades of expertise, Scott A. Snyder and Brad Glosserman investigate the roots of this split and its ongoing threat to the region and the world.

Roughly once a year, the managing director of the International Monetary Fund (IMF), the U.S. Treasury secretary, and, in some cases, the finance ministers of other G7 countries get a call from the f…