The Forces Behind South Korea’s and Japan’s Thaw

For years, the forces driving South Korea and Japan apart, deeply rooted in bitter history, had seemed too strong to overcome despite repeated efforts and the urging of their mutual ally, the United States.

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 that has broken several promises, including treaty agreements that were designed to salve historical wounds.

But the advent of two new administrations in the neighboring countries — President Yoon Suk Yeol in South Korea, and Prime Minister Fumio Kishida in Japan — has led to a rapid thawing of relations.

In March, the two countries began taking steps to address a long-festering dispute over wartime forced labor. In April, South Korea restored Japan’s status as a preferred trading partner, prompting Tokyo to start the process of restoring the same status for South Korea. And Mr. Yoon drew notice in his home country after declaring that Japan must no longer be expected to “kneel because of our history 100 years ago.”

Now, Mr. Kishida is making a personal visit to South Korea, in a meeting that is being closely watched for new signs of progress. Here are some of the global forces behind their mutual outreach.

Tension with China and Russia

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, and as Russia’s invasion of Ukraine raises alarm about a new era of militarization.

Both countries supported the Biden administration’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.

Mr. Kishida now calls South Korea “an important neighboring country that we should work with.” And Mr. Yoon has urged South Koreans 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’s nuclear 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.

Vulnerable global supply chains

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.

Recently, though, 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.

Growing fears about Taiwan

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 provides by keeping a large military presence in both nations. 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 improve its ability to defend Taiwan.