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, Brad Glosserman and Scott A. Snyder investigate the roots of this split and its ongoing threat to the region and the world.

Glosserman and Snyder isolate competing notions of national identity as the main obstacle to a productive partnership between Japan and South Korea. Through...