Much has been written about the U.S.-Japan-China and U.S.-Japan-Russia trilateral relationships and their respective impact on the broader Asia-Pacific security environment. There is, however, another three-way relationship that attracts far less attention but which can have an equally far-reaching impact on regional security. I refer to U.S./Japan relations with the Republic of Korea today and with a unified Korea at some (unpredictable) point in the future.