In December 2018, a South Korean destroyer allegedly locked its targeting radar on a Japanese surveillance aircraft. Although details of that incident remain bitterly contested, the controversy captures well the suspicions and ill-will that have engulfed Japanese-South Korean relations. The tarring of that military-to-military relationship — one that has historically served as a shock absorber for difficulties between the two governments — is particularly troubling for the United States. The challenge of deterring a nuclear-armed North Korea is more severe if its two closest allies in Northeast Asia are more concerned about each other than their common adversary.

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