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U.S.-China relations are frayed, with Washington openly critical of Beijing’s apparent refusal to get tough with Pyongyang. The U.S. last month ignored Chinese disapproval to hold joint maritime exercises with South Korea in the Yellow Sea.

The day Mr. Gates arrived in China, he was greeted with the first-ever test flight of a Chinese stealth warplane, though he accepted Mr. Hu’s assurance that the flight was unrelated to his trip.

Recent Northeast Asian tensions have not been limited to the Korean peninsula.

Japan-China relations are strained following a clash between Chinese fishing boats and Japanese coast guard vessels off disputed islands in September. During that dispute, U.S. officials publicly raised the U.S.-Japan defense treaty, noting that it covers areas “administered” by Japan.

Against the backdrop of these developments, Tokyo and Seoul are cautiously discussing defense ties. And Navy Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, suggested three-way defense exercises during a trip to Korea last month.

Japanese Defense Minister Toshimi Kitazawa held talks with Korean counterpart, Kim Kwan-jin, this week on a two-day trip to Seoul, though the meetings hardly delivered stellar results: The only agreements reached were to continue talks on sharing intelligence and exchanging hardware.

Japanese Foreign Minister Seiji Maehara arrives in Seoul Saturday for a visit during which common approaches toward North Korea will be discussed.

While the two East Asian democracies share concerns over China and have separate defense treaties with the United States, bitter historical memories — Japan colonized Korea between 1910-1945 — and a disputed island in the Sea of Japan are highly emotive issues for South Koreans.