Rising diplomatic tensions between South Korea and Japan are putting American security interests at risk. Yet the United States government appears detached, unable to facilitate a rapprochement between its two allies. This is a critical moment because a South Korea-Japan intelligence-sharing agreement, aimed at North Korea, is due to lapse on 22 November this year. If Japan and South Korea come to regard the United States as an unreliable security partner, both countries may seek to develop independent nuclear capabilities, not an outcome that many would welcome. Rising diplomatic tensions between South Korea and Japan are putting American security interests at risk. Yet the United States government appears detached, unable to facilitate a rapprochement between its two allies. This is a critical moment because a South Korea-Japan intelligence-sharing agreement, aimed at North Korea, is due to lapse on 22 November this year. If Japan and South Korea come to regard the United States as an unreliable security partner, both countries may seek to develop independent nuclear capabilities, not an outcome that many would welcome.