South Korea and Vietnam established diplomatic relations only twenty years ago. Today these former adversaries enjoy unexpectedly cordial and rapidly expanding bilateral ties. Leaders of the two nations--perceiving broadly shared interests and no fundamental conflicts--seek to leverage their subregional influence on behalf of common or complementary policy goals. Today they often profess a "middle power" identity as they explain their foreign policy in terms of such classical middle power goals as regional peace, integration, and common goods.

Broadly similar in many respects, South...