This piece is one of 12 contributions to KEI’s special project on South Korea’s nuclear armament debate that will run on The Peninsula blog over the next month. The project’s contributors include young, emerging, and mid-career voices, examining the debate from a historical, a domestic, and an international perspective. On Wednesday, March 15, KEI will host a conference featuring our various contributors’ work at our Washington, D.C. office and launch a compilation of all the pieces in a single, special KEI publication.
As a close geographic neighbor and diplomatic partner, Japan has a strong interest in the course of Korea’s debate to develop an indigenous nuclear capability. Japanese leaders have long grappled with the promises and perils of such weapons, and which have gained even more relevance given the invasion of Ukraine and tensions in the Taiwan Strait. If Korea chose to pursue a nuclear weapon, it is likely that Tōkyō would initially signal its disapproval with diplomatic and economic sanctions, but this may dissipate as Japan has also been pragmatic about working with the partners it has, and not the partners it wants. However, such a radical change in Korean policy would also complicate an already sensitive relationship.
As the only country to have come under a nuclear attack in wartime, Japan has officially been a strong opponent of nuclear weapons in the international system. In the postwar period, it signed most of the major international non-proliferation treaties. Japan signed the Partial Test Ban Treaty (PTBT) in August 1963, which prohibits the testing of nuclear weapons in the air, space, and sea. It also signed the Treaty on the Non-Proliferation of Nuclear Weapons (NPT) in February 1970, which sanctions the peaceful development of nuclear technology and prohibits non-nuclear weapons states from developing such weapons. Japan also signed the Comprehensive Nuclear-Test-Ban Treaty (CTBT) in September 1996, which prohibits nuclear explosions for any purpose. This treaty has not yet entered into force.
Japanese opposition to nuclear proliferation is also reflected in several domestic laws and norms. Chief among these is Article 9 of the constitution, which states that “land, sea, and air forces, as well as other war potential, will never be maintained” by Japan. Article 2 of the Atomic Energy Basic Act, enacted in December 1955, specifies that “research, development and utilization of nuclear energy is limited to peaceful purposes.” While not codified in law, the Japanese government has also been guided by the Three Non-Nuclear Principles. These were first articulated Prime Minister Eisaku Satō in remarks to the Diet in December 1967, and they include “not possessing, not producing and not permitting the introduction of nuclear weapons, in line with Japan’s Peace Constitution.”
This opposition to nuclear weapons has been maintained by Prime Minister Fumio Kishida, who represents a constituency in Hiroshima and is well-acquainted with the horrors of atomic warfare. He chose Hiroshima City to host the next G7 summit later this year, given Russia’s loose rhetoric of nuclear use. “It is important to convey the reality of the atomic bombings to the world, including the G7 leaders, as the starting point for all efforts toward nuclear disarmament,” he said earlier this month. Prime Minister Kishida also became the first Japanese leader to attend the NPT Review Conference last year, where he introduced the “Hiroshima Action Plan” to reduce nuclear weapons. It consists of five parts: recognition never to use nuclear weapons; transparency of nuclear capabilities; reducing nuclear stockpiles; promoting the peaceful use of nuclear energy; and promoting visits to Hiroshima and Nagasaki. “I cannot but admit that the path to a world without nuclear weapons has become even harder,” said Prime Minister Kishida in his remarks to the Conference. “Nevertheless, giving up is not an option.”
But this commitment to non-nuclear weapons has never been absolute. Scholars have documented how postwar Japanese leaders have flirted with nuclear weapons despite popular disapproval. The earliest such remark was in May 1957, when then Prime Minister Nobusuke Kishi suggested that small nuclear weapons would not violate the Japanese constitution. Prime Minister Satō himself, the man who articulated the Three Non-Nuclear Principles, is reported to have told President Lyndon Johnson that Japan needed nuclear weapons in order to deter China.
This pattern continued in the Heisei and Reiwa eras, under numerous governments including that of Prime Minister Shinzō Abe. In comments echoing his grandfather, he suggested that small nuclear weapons would not violate the Japanese constitution. He was also criticized when he omitted the Three Nuclear Principles during a speech commemorating the 70th anniversary of the atomic bombing of Hiroshima. They eventually were included in his following speech in Nagasaki. Even under Prime Minister Kishida, Tōkyō still has not signed the Treaty on the Prohibition of Nuclear Weapons, which calls on states not to develop, manufacture, or use nuclear weapons, and entered into force in January 2021. Although committing Japan to realizing a world without nuclear weapons, a foreign ministry spokeswoman said in 2022 that this cannot be achieved without the buy-in of states with such weapons. “The truth is that none of the nuclear-weapon states has participated in the treaty,” said Ministry of Foreign Affairs Spokeswoman Hikariko Ono.
At this time, Japanese conversations on nuclear weapons have not focused on South Korea. Mainstream media sources have reported on recent Korean public opinion polls, but few have raised concerns for their implications for Japan. In recent weeks, Japanese officials have engaged Korean officials on at least two occasions. But in the case of Chief Cabinet Secretary Hirokazu Matsuno and Director-General Takehiro Funakoshi of the Asian and Oceanian Affairs Bureau at MOFA, they have focused on the issue of compensation for former laborers. It seems that for the moment, Japanese officials are focused on the historical issues with Korea, and not the domestic Korean debate on nuclear weapons.
Despite this silence, Japan’s reaction to India’s nuclearization provides some insight to how it would react to Korea. Over two days in May 1998, India conducted a total of five nuclear tests that came to be known as Pokhran-II from its location in the desert. The Indian diplomat S. Jaishankar would later recall the “swift and exceptionally harsh” response by Japan. Economic sanctions included freezing existing Japanese aid grants and loans to India, as well as pushing for a review of loans made by international organizations. Diplomatic sanctions included recalling the Japanese ambassador to Tōkyō, and using international forums like the G8, UN Security Council, and the International Atomic Energy Agency to also condemn India’s tests.
Despite this initial response, Japan would eventually relent in its attempts to punish India. Two years after Pokhran-II, Japanese Prime Minister Yoshiro Mori announced in 2000 that he would visit South Asia, with a stop scheduled in India. In a readout of his meeting with Indian Prime Minister Atal Bihari Vajpayee, Prime Minister Mori emphasized Japanese concerns about Indian nuclear weapons. But in exchange for India’s unilateral moratorium on nuclear testing and work towards joining the CTBT, Japan announced support for two Indian infrastructure programs. “Time will reassure Japan that there is no automatic spread effect to the Indian nuclear tests and that its own immediate security environment is not adversely affected,” writes Mr. Jaishankar. “Recent history has frequently demonstrated Japan’s pragmatism and there is no reason to suppose that this would not extend to India.” Indeed, the early decades of the 21st century have borne out Mr. Jaishankar’s optimism, and the India-Japan relationship has become one of the strongest in the Indo-Pacific region.
India’s example would be a best-case scenario from Korea’s perspective but may not be the most likely given the differences in NPT membership between India and South Korea. An announcement by Seoul that it would pursue a nuclear weapon would be met with official condemnation from the Japanese government, and may also involve the recall of diplomatic representation. Economic sanctions and pressure from other international fora would follow. But Japanese pragmatism may eventually win the day should that initial pressure not convince Seoul to reverse course, and see a gradual resumption of relations. During the Cold War, Japan chose not to seek nuclear weapons after the Soviet Union and mainland China developed such capabilities. Assuming South Korea maintained its relationship with the United States, it is conceivable that Tōkyō would come to see that these nuclear weapons pose no threat to Japanese territory.
If South Korea chose to seek an indigenous nuclear weapons capability, it is unlikely Japan would follow suit. Japanese memories of Hiroshima and Nagasaki remain strong, complicating the political costs of Tōkyō’s nuclearization. It is telling that even when the late former Prime Minister Abe made waves for his comments on nuclear weapons in February 2022, it was not an outright endorsement for their acquisition. Japan “should not put a taboo on discussions [my emphasis] about the reality we face,” he said during a television program on the Russian invasion of Ukraine. This focus on the discussion for nuclear sharing, but not nuclear weapons themselves, is also seen in the results of a public opinion poll by the conservative Sankei Shimbun and Fuji News Network. The outlets reported that 62.8% of respondents opposed nuclear sharing with the United States, but maintained that it should still be discussed. While Japan may be more comfortable talking about nuclear weapons, there is little appetite to actually do so.
Instead, Japan has been more focused on conventional ways to defend itself. One of the more significant changes in Japan’s latest national security strategy documents was approval for counterstrike capabilities against enemy bases. The conservative Yomiuri Shimbun ran an editorial January that said South Korean nuclear weapons would not help the North Korea problem, and reverse progress towards denuclearization. “A realistic measure is to enhance the credibility of the U.S. nuclear umbrella, such as by frequently conducting joint U.S.-South Korean military drills using U.S. bombers that can carry nuclear weapons,” the editorial said. While Japan and Korea both share concerns about regional security challenges and the relationship with the United States, the former finds it more acceptable to focus on conventional capabilities, rather than weapons of mass destruction.
While Japanese officials have remained quiet about Korea’s nuclearization, such an event will likely push the bilateral relationship to new lows. From Japan’s perspective, Korea in recent years has continually gone back on its word and abrogated bilateral agreements. The legitimacy of the 1965 treaty normalizing relations was called into question under the Moon administration, which also dissolved the 2015 comfort women deal. While historical issues are beyond the scope of this article, Korea fatigue in Japan is a real challenge for Korean policymakers. Seoul’s decision to withdraw from the NPT would be yet another agreement Korea signed that it reneged on, and be viewed as a betrayal of the rules-based liberal international order that has benefited both Japan and Korea. The government in Seoul should not expect to receive the same reception in Japan as India did.
As the South Korean public engages in speculation over developing its own nuclear weapon, Japanese policymakers have focused their attention on other long-standing issues in the bilateral relationship. Perhaps this demonstrates reticence to comment on a domestic issue, or a belief that Seoul will continue to be a responsible member of the international community and abide by its non-proliferation obligations. If Korea made a decision to pursue nuclear weapons, there is a chance Japan could come to terms with it as it has in Russia, China, and perhaps reluctantly in North Korea. But that decision has the potential to rupture Korea-Japan relations in a way that has not been seen before.
Terrence Matsuo is a Non-Resident Fellow at the Korea Economic Institute of America. The views expressed here are the author’s alone.
Photo from Shutterstock.