South Korea and Japan have been the two most important allies of the United States in East Asia for decades, and it has long troubled Washington that the pair could not get along.
South Koreans say Japan never properly apologized or atoned for its brutal colonial rule of the Korean Peninsula from 1910 to 1945. To the Japanese, South Korea has often been an untrustworthy neighbor who has broken several promises, including treaty agreements that were designed to salve historical wounds.
In recent years, the United States has found the need for a diplomatic rapprochement between its East Asian allies more pressing than ever as it tries to mobilize like-minded partners to deal with common threats, such as the Russian invasion of Ukraine, China’s economic and military ambitions and North Korea’s growing nuclear weapons program.
Against this backdrop, ties between Seoul and Tokyo have started to thaw. In March, the two countries began taking steps to address a long-festering dispute over wartime forced labor. This week, South Korea restored Japan’s status as a preferred trading partner, and the South Korean president, Yoon Suk Yeol, drew notice in his home country after declaring that Japan must no longer be expected to “kneel because of our history 100 years ago.”
As Mr. Yoon visits Washington this week for a state dinner on Wednesday and an address to Congress the following day, President Biden and other American officials will discuss ways to continue the momentum toward détente. Here is why it is crucial to Washington’s strategy in Asia and beyond.
Alliances are crucial in a shifting world.
The United States has tried to persuade its allies in the Indo-Pacific to cooperate more closely by introducing a bevy of partnerships, such as the Indo-Pacific Economic Framework for Prosperity, the four-nation “Quad” consultative body, the AUKUS security pact, the Chip 4 alliance and the Partners in the Blue Pacific initiative.
A strong bilateral partnership between Japan and South Korea has long been on Washington’s wish list, but the troubled relationship has kept it from happening.
Now, both Tokyo and Seoul are moving to align themselves more closely with Washington as China promotes an alternate vision of the world in which the United States has less power.
Both countries supported Washington’s “free and open” Indo-Pacific vision, attending a NATO summit meeting last summer where leaders condemned Russia’s invasion of Ukraine and expressed concern about China’s threat to undermine the international rule-based order.
Both countries have realized that the fast-changing, geopolitical environment has created challenges they cannot deal with alone. The joint maneuverings by Chinese and Russian military aircraft near South Korean and Japanese airspace in recent years helped drive that message home.
Prime Minister Fumio Kishida of Japan now calls South Korea “an important neighboring country that we should work with.” President Yoon Suk Yeol of South Korea has urged his country to no longer regard Japan as “a militaristic aggressor of the past” but as “a partner that shares the same universal values.”
The trilateral relationship with South Korea and Japan “is central to our shared vision of a free and open Indo-Pacific region, which is why I, along with other senior Department colleagues, have invested so much time and focus on this critical partnership,” Secretary of State Antony J. Blinken said in March.
North Korea is expanding its arsenal.
North Korea’s growing nuclear and missile threat was an incentive for Seoul and Tokyo to recognize the strategic value of building up trilateral cooperation with the United States. In recent months, North Korea has not only fired missiles over Japan, but also threatened a nuclear attack on South Korea.
South Korea has never been formally allied with Japan and has been reluctant to cooperate militarily with the country beyond humanitarian search-and-rescue missions on the high seas. But they are expanding military cooperation now, mainly because of North Korea.
When the leaders of the United States, Japan and South Korea met in Phnom Penh, Cambodia, last November, they agreed to share real-time North Korean missile warning data. The three nations have also expanded trilateral missile defense and other military exercises in recent months.
One of the steps Seoul took to mend ties with Tokyo in March was to formally reinstate a bilateral military intelligence-sharing agreement that helps the two neighbors guard against North Korean missiles. At the height of the dispute over wartime forced labor in 2019, Seoul announced plans to terminate the accord.
Global supply chains are more vulnerable.
That same year, 2019, Japan imposed restrictions on exporting chemicals essential to South Korea’s semiconductor industry. Seoul filed a complaint against Tokyo with the World Trade Organization. Both nations removed each other from their so-called white list of preferential trade partners.
But in an atmosphere of increasing threat to global supply chains because of war and geopolitical tension between the United States and China, in particular, Japan and South Korea are moving toward better mutual support.
Last month, Tokyo and Seoul agreed to withdraw those export controls, and Seoul withdrew its W.T.O. complaint. Seoul and Tokyo also agreed to start an “economic security dialogue” to discuss cooperation in key technologies and supply chains. Mr. Yoon’s government recently expressed hopes of attracting Japanese companies to a $228 billion semiconductor complex South Korea plans to build near Seoul by 2042.
South Korea is the world’s leading producer of memory chips, and Japan supplies tools and materials essential to chip-making. Last year, Washington proposed the so-called Chip 4 Alliance with the two allies and Taiwan to keep China at bay in the contest for global semiconductor supply chains.
Tension with China over Taiwan is rising.
Seoul, Tokyo and Washington share a strong common interest in keeping peace and stability in the Taiwan Strait.
Security analysts fear that China might attempt to invade Taiwan, similar to Russia’s full-scale invasion of Ukraine. If that happened, some experts warn that North Korea might take the opportunity to start a war on the Korean Peninsula and realize its own territorial ambitions.
Such a move would open two simultaneous battlefronts for the American military in the region.
“If a clash erupts in the Taiwan Strait, the United States will demand various cooperation from its allies and partner nations,” Kim Han-kwon, a professor at the Institute of Foreign Affairs and National Security in Seoul, wrote in a paper in February. “It sees its bilateral alliances with South Korea and Japan, in particular, as key regional strategic assets in connection with the Taiwan Strait.”
Japan and South Korea have been able to thrive economically in part because of the security the United States proes by keeping a large military presence in both nations. Washington has also vowed to protect its allies through “extended deterrence,” a commitment to using the full range of American weapons — including nuclear capabilities — in the event of conflict.
Now, the United States wants all its allies to play a bigger role in regional defense.
In addition to South Korea and Japan, Washington has recently moved to strengthen its military ties with Australia, India and the Philippines to counterbalance China’s influence in the region and to bolster its ability to defend Taiwan.
