Commentary For years, American diplomats and military leaders have been asking the South Koreans and the Japanese: “Can’t we all just get along?” The answer is no—at least according to Lee Jae-myung, the ruling Democratic Party’s candidate for next year’s South Korean presidential election. In a recent interview, he ruled out the possibility of a U.S.-South Korea-Japan military alliance, adding that South Korea “needs to be prepared in the event [that] Japan’s dream of continental expansion erupts militarily.” The headaches continue for the Americans. Common Interests, but Still Antagonists Logic suggests Japan and the Republic of Korea (ROK) should cooperate well. Emotion—sometimes stoked by outside actors with their own agendas—ensures that they don’t. Leaving aside the threat from China, South Korea and Japan face common and immediate threats from North Korea. The ROK faces a serious conventional and nuclear threat, and Seoul is within artillery range of North Korean batteries. …