SEOUL, SOUTH KOREA — The United States, Japan and South Korea last week held what in some ways could be seen as their most important joint military exercise ever — and an indication of enhanced future cooperation.

The inaugural Freedom Edge drill involved a U.S. aircraft carrier and multiple Japanese and South Korean ships and planes, mirroring other recent trilateral exercises held since the three countries intensified defense cooperation.

But, importantly, this drill for the first time took place across multiple domains, including land, sea, air and cyber — a crucial step toward allowing the countries’ militaries to work together more seamlessly and in a wider range of warfighting scenarios.

The drill reflects a bigger effort by Washington, Seoul and Tokyo, which are trying to advance cooperation toward a more formalized stage that will be harder for future leaders to overturn.

In recent months, the three countries have not only expanded the frequency of their engagement but also taken steps to ensure that it lasts — an attempt to solidify a partnership that could reshape northeast Asian geopolitics.

The steps include establishing a regular pattern of joint military exercises, activating a channel for sharing real-time data on North Korean missile launches, and exploring the creation of a permanent office to boost coordination.

During meetings among senior officials, the countries have also increasingly emphasized shared values for the region, such as a “free and open Indo-Pacific,” in the hopes of providing a more durable foundation for cooperation.

The moves attempt to fulfill the vision laid out in August, when U.S. President Joe Biden, Japanese Prime Minister Fumio Kishida and South Korean President Yoon Suk Yeol held the first standalone summit among the leaders of the three countries.

South Korea key

A primary goal of the so-called Camp David summit was to create a framework for collaboration that could withstand domestic political fluctuations in each country.

A major concern was South Korea, where commitment to the trilateral partnership has often wavered. Those efforts appear to be yielding progress, according to a growing number of South Korea-based observers.

“Cooperation is now entering a level of institutionalization that will make it considerably more difficult for future administrations in Seoul to change,” said Jeffrey Robertson, a professor of diplomatic studies at Seoul’s Yonsei University.

The depth of trilateral ties has long hinged on whatever government is in power in South Korea.

The South Korean left opposes closer cooperation with Japan without more steps by Tokyo to atone for atrocities committed during its 1910-1945 colonization of Korea.

South Korea’s leaders also have been reluctant to sign up for any multilateral efforts that anger China, the military and economic giant that lies just beyond its border.

Changing views

South Korea’s outlook toward its neighbors, however, appears to be shifting.

Opinion polls suggest that South Korean perceptions of China have declined precipitously, as Beijing becomes more authoritarian at home and more assertive in expanding its regional influence.

Meanwhile, views on Japan appear to be improving, especially among young people.

South Korea’s national security establishment has also expressed growing fears about North Korea, which has rapidly expanded its nuclear arsenal and become much more hostile toward Seoul.

For Yoon, a conservative who took office in 2022, the solution was to align his country more closely with the United States. Yoon also mended ties with Japan, quickly accelerating trilateral cooperation.

Reasons for optimism

The big question is whether Yoon’s approach will outlast his presidency, given that his predecessor, the left-leaning Moon Jae-in, reversed many of the Japan-friendly policies of previous administrations.

Peter Lee, a research fellow at the Seoul-based Asan Institute for Policy Studies, a conservative research group, says he is optimistic. Although he concedes that South Korean public attitudes toward Japan remain generally unfavorable, he points to opinion polls conducted by his organization suggesting consistently strong support for South Korean participation in U.S.-led multilateral initiatives.

“This suggests that future ROK presidents will struggle to withdraw or terminate their participation in these partnerships, at least for populist purposes,” Lee said.

Another potential deterrent is that each step toward formalizing trilateral engagement adds a layer of commitment, making it politically riskier for any future South Korean administration to reverse, many observers say.

Not so fast

Others think that Yoon’s policies are on much shakier ground.

Moon Chung-in, a senior foreign affairs adviser in multiple left-leaning governments, rejected the notion of a permanent change in South Koreans’ views toward their neighbors.

“Yoon and Biden do not see this. But Japanese political leaders are well aware of the volatility, and that’s why they are not making major concessions,” Moon said.

Seoul-Tokyo ties can “easily degenerate,” Moon maintained, unless Japan takes further steps to address unresolved historical disputes.

Many in South Korea also worry that enhanced trilateral cooperation could provoke a counter-reaction from U.S. foes in the region, ultimately leading to increased instability.

As evidence, they cite last month’s decision by North Korea and Russia to restore a Cold War-era mutual defense treaty — a move both sides described as necessary to counter U.S. moves in Asia.

If the regional security dilemma worsens, some fear that China could eventually respond by enhancing its own security cooperation with North Korea and Russia.

For many South Korean liberals, the best way to avoid such a scenario with China is to take a more cautious approach to Japan and the United States.

“China is near and powerful,” Moon said. “What other options do we have but to maintain good relations? This is common sense.”