One of the problems a great power faces is how to handle situations where two or more allies quarrel and adopt antagonistic policies towards each other. The United States faces that challenge now, as Japan and the Republic of Korea (ROK) engage in escalating disputes involving both economic and security issues. Their spats could scarcely come at a worse time. President Trump is pursuing a delicate policy of rapprochement with North Korean leader Kim Jong-un. For that initiative to have even a reasonable chance of success, both Tokyo and Seoul need to be supportive and not undermine Washington’s approach.

This is hardly the first time that an American leader has dealt with headaches caused by allies or security dependents that seem to loathe each other. Since Turkey and Greece joined NATO in 1952, they have frequently pursued conflicting foreign policy goals—most notably with respect to the Bosnia and Kosovo conflicts in the 1990s. Worse, they have nearly come to blows on several occasions. The worst incident occurred in 1974 when the military junta ruling Greece helped unseat the moderate president of neighboring, majority-Greek Cyprus in a bid to orchestrate a merger of the two countries. Turkey responded by invading Cyprus on the pretext of protecting the Turkish ethnic minority there, and proceeded to occupy nearly 40 percent of the island, expelling the Greek inhabitants. Washington was barely able to prevent a war. Even before the Cyprus incident, Turkey had made a habit of sending its warplanes into Greek airspace, stoking tensions. These antagonisms continue even now: there were some 36 violations on a single day in December 2018.

A desire to preserve their security ties with the United States against a larger, more powerful potential aggressor was the major factor that inhibited Greece and Turkey from letting their own rivalry spiral out of control. A similar situation exists with Japan and the ROK. During the Cold War, worries about North Korea, China, and the Soviet Union compelled a degree of unity as Washington put both countries behind the U.S. security shield. In the post-Cold War years, concerns over Pyongyang’s volatile behavior muted the animosity between Japan and South Korea.

Nevertheless, there is no love lost between these two countries. Many Koreans have never forgiven Japan for the abuses Tokyo committed between 1910 and 1945 as Korea’s colonial master. Forcing young Korean women into sexual servitude to the Japanese military was only the most egregious of the offenses, and Tokyo’s reluctance to apologize for that outrage and compensate the victims has exacerbated the resentment among South Koreans.

Other quarrels flare up from time to time. One is the simmering territorial dispute over a cluster of small islands that the Japanese call Takeshima and the Koreans call Dokdo. Although the tangible stakes are minimal, the symbolism has resulted in dangerous posturing (including military maneuvers) by both sides on several occasions. The latest petty display erupted when Seoul circulated a map for the upcoming 2020 Olympics showing the islands as part of the ROK. Japanese officials responded with shrill protests.

As both Japan and the ROK emerged as major players in the global economy, bilateral economic spats became more frequent. Indeed, the current surge of quarrels began primarily in the economic arena. In recent weeks, trade tensions have become especially acute, with Japan moving to eliminate South Korea from the list of countries enjoying minimum restrictions. Seoul has taken that dispute to the World Trade Organization, where the ROK position was greeted with a lack of enthusiasm bordering on indifference.

Such quarrels and their potential for poisoning relations between two allies undoubtedly concern U.S. officials. But the growing gap between the Japanese and Korean positions regarding North Korea may prove to be a more serious worry. With the election of President Moon Jae-in, the ROK has pursued a highly conciliatory policy toward Pyongyang. Indeed, Seoul has prodded the Trump administration to back away from its initial confrontational approach to North Korea. Moon has helped facilitate Trump’s summit meetings with Kim and avidly cheered the easing of tensions.

Japan’s reaction, while officially supportive of Trump’s diplomacy, has been noticeably more subdued. As the second Trump-Kim summit in Hanoi approached in February 2019, Prime Minister Shinzo Abe’s government exhibited distinct signs of uneasiness that it was being bypassed and that important Japanese security interests might be sacrificed. Japan’s military and diplomatic establishments appear less than thrilled about Washington’s budding rapprochement with Pyongyang. The ROK’s own warming relationship with North Korea is generating noticeable frictions between Seoul and Tokyo. Recently, Abe implied that Moon’s government was not enforcing sanctions against Pyongyang, which drew an angry reaction.

Managing the contentious relationship between Japan and South Korea may prove even more difficult for the United States than dealing with the habitual tensions between Greece and Turkey. Athens and Ankara are at least both members of NATO and have treaty obligations to each other. Japan and South Korea are linked to the United States through separate mutual defense treaties, but there are no official multilateral defense ties. Japanese-ROK security initiatives have remained tentative and tepid. Indeed, Washington’s repeated efforts to promote robust, official security cooperation have been met with a lack of enthusiasm.

All this means the Trump administration will go into the next phase of its dealings with Pyongyang with considerable uncertainty about the unity and common purpose of its two key East Asian allies. In an increasingly dangerous world, that’s a worrying prospect.

Ted Galen Carpenter, a senior fellow in security studies at the Cato Institute and a contributing editor at The American Conservative, is the author of 12 books and more than 800 articles on international affairs.