Donald Trump’s desire to put “America First” has fostered new disputes between the United States and its allies. In Asia, old rivalries are also roaring back.

Ties between Japan and South Korea — two of Washington’s closest security partners — have arguably turned their most hostile in more than half a century over a series of diplomatic disputes. Now, there are signs that the feud, fueled by disagreements over Japan’s colonization of the Korean Peninsula decades ago, is beginning to damage their economic and military relations.

During previous nationalistic flare-ups, U.S. administrations normally intervened to make sure such grudges did not spin out of control. Not any more.

“There’s no leadership from above in the U.S. administration to act in the way we have in the past,” said Daniel Sneider, a lecturer in international policy at Stanford University and author of books on Northeast Asian relations. “At moments like this, it’s been the role of the U.S. to sometimes quietly step in and help to restore communication and sometimes to find solutions.”

The episode in Northeast Asia illustrates how Trump’s skepticism of traditional U.S. alliances and preoccupation with a rolling series of political crises in Washington may be quietly reshaping the postwar geopolitical landscape. The U.S. State Department didn’t respond to requests for comment last Thursday and Friday.

A second summit planned for later this month between Trump and North Korean leader Kim Jong Un has brought more U.S. attention to the region, with U.S. special envoy Stephen Biegun planning to meet his North Korean counterpart this week.

While ties between Japan and South Korea run deep — each is the other’s third-largest trading partner — they’re laden with centuries of grievances, especially Tokyo’s 1910-1945 colonization of the peninsula. Disputes over whether Japan has sufficiently atoned for its actions returned to the fore after Moon Jae-in won the South Korean presidency in 2017 and pushed back against Prime Minister Shinzo Abe’s efforts to put the war disputes to rest.

In recent weeks, Japan has reacted angrily to South Korean court efforts to seize assets from companies found to have used forced Korean labor during the colonial era, saying the move violates the 1965 treaty that established bilateral diplomatic relations. They’ve also sparred over the issue of “comfort women” who provided sex, including those who did so against their will, for Japanese troops before and during World War II, and with Moon vowing after the death of a leading comfort woman campaigner last week to do everything in his power to “correct the history.”

Perhaps most consequential to the U.S. is the deteriorating ties between the Japanese and South Korean militaries — something that could undermine American efforts to counter a rising China.

The defense hierarchies in both countries have accused the other of endangering their personnel after Japan said a South Korean naval vessel used a weapons-targeting radar on a Maritime Self-Defense Force plane in December.

After Defense Minister Takeshi Iwaya highlighted the radar incident during a visit to the base where the plane was stationed, his South Korean counterpart, Jeong Kyeong-doo, ordered the navy to “deal sternly” with any other low-flying planes. Kyodo News and other media have reported that defense exchanges are being postponed, increasing the potential for misunderstanding during unplanned encounters.

With nationalist sentiments running high, neither Abe nor Moon have much domestic incentive to settle.

A poll by the Nikkei financial newspaper published Jan. 28 found that 62 percent of Japanese respondents supported a tougher stance against South Korea over the radar incident. That compared with 24 percent who said the Abe government should watch developments cautiously.

“We cannot see where the bottom is,” said Lim Eunjung, an assistant professor at the College of International Relations at Ritsumeikan University in Kyoto Prefecture.

Although Tokyo and Seoul have managed to keep their feuding from turning violent over the years, economic risks are growing, including possible Japanese countermeasures against South Korean companies.

The number of South Koreans visiting Japan fell 5.5 percent year on year in November to 588,000, according to the Japan National Tourism Organization, even as the overall number of inbound tourists rose 3.1 percent.

In the past, the U.S. has used its leverage as chief security guarantor to keep the rivalry in check, helping to broker their 1965 treaty despite domestic opposition on both sides.

Barack Obama’s administration played a key role in getting Tokyo and Seoul to sign a comfort women pact in 2015 and a military intelligence-sharing agreement in 2016 — high-water marks for the relationship.

Trump, however, has shown little interest in the alliance since abandoning his military pressure campaign against North Korea last year, focusing instead on U.S. trade deficits and military expenditures with both countries.

The U.S. leader skipped a pair of Asian summits in November to focus on the midterm elections and didn’t hold a trilateral meeting with Abe and Moon at the subsequent Group of 20 gathering in Argentina.

Some signs of possible diplomatic efforts by lower-level U.S. officials have emerged in recent days.

Ambassador to South Korea Harry Harris visited the Defense Ministry in Seoul early last week, while other U.S. officials, in calls with their Japanese counterparts, have emphasized the importance of the three countries working together on North Korea.

But even on key issues the two countries appear to have little common ground.

While South Korea settled its trade negotiations with the U.S. and wants to encourage Trump’s rapprochement with Kim, Japan has yet to start trade talks with the U.S. and favors a more cautious approach with Pyongyang.

“Political leaders on both sides must realize that the damage from deteriorating Japan-South Korea ties fundamentally undermines the U.S. alliance system in East Asia,” said Yuki Tatsumi, co-director of the East Asia Program at the Stimson Center in Washington.

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