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The United States is moving forward with the nuclear-armed sea-launched cruise missile (SLCM-N) development. From a Japanese perspective, the political risk of the SLCM-N deployment for the Japan-US alliance merits attention due to the way it would force Japanese policymakers to reckon with the impact on the Three Non-Nuclear Principles of not possessing, not producing, and not permitting the introduction of nuclear weapons into the country, which Japan has upheld since 1968. Attention to these Principles faded after the US global withdrawal of tactical nuclear weapons after the Cold War, but the “no introduction” issue in particular could resurface as the United States operationalises the SLCM-N.

This policy brief elaborates on the political risk associated with the SLCM-N while providing an overview of Japanese perspectives on this nuclear issue.

The United States stationed nuclear weapons in East Asia during the Cold War, and through a “secret nuclear deal” between Japan and the United States, Japan gave de facto tacit approval for US nuclear transits, even after the Three Non-Nuclear Principles were adopted in 1971. Today, Japanese administrations have maintained that whether or not to allow future nuclear transits is up to the government to decide.

From a Japanese security perspective, advocates argue that the SLCM-N would enhance US extended nuclear deterrence by adding a prompt and survivable low-yield nuclear option to the existing US nuclear arsenal, that could strike against Chinese and North Korean targets. Opponents argue that existing capabilities can fill this role, and that the deployment of the SLCM-N could heighten the risk of inadvertent nuclear escalation due to at-launch ambiguity, and undermine the US conventional deterrent.

The deployment of the SLCM-N also comes with political risks, most prominently from domestic backlash against any Japanese government that would allow nuclear transits. The Japanese government is thus faced with five options. It could:

1. Seek the cancellation of the SLCM-N program. While this option exists, it is unlikely that a Japanese government would pursue it, as it also comes with political costs for the alliance.
2. Uphold the Principles and ensure no nuclear transits. Tokyo could use the “New Zealand” model, forbidding nuclear transits entirely, but this might come at the cost of the entire alliance.
3. Maintain the Principles but tacitly allow nuclear transits. This status quo option is less tenable than in the past, as the history of secret nuclear transits in Japan is now common knowledge.
4. Settle the “no introduction” interpretation issue and permit nuclear transits. The government could reinterpret the “no introduction” principle, at the risk of huge domestic backlash.
5. Remove the no introduction principle and permit nuclear transits. The government could also remove the principle entirely, which would not only trigger domestic backlash, but potentially open up for the deployment of US nuclear weapons on Japanese soil.

The policy brief makes three recommendations:

Do not remove the no introduction principle, and ensure public debate on its interpretation: The risks of removing the no introduction principle (Option 5) are too high and is not advisable from a Japanese security perspective.

Conduct a comprehensive and independent policy review: The United States and Japan should undertake a joint comprehensive and independent policy review on the SLCM-N to weigh its political risk against perceived security benefits, along with other costs such as nuclear risks and tradeoffs with conventional deterrence capabilities. To do so, they should commission a review by independent Japanese and US experts who address not only defense and security perspectives but also arms control and disarmament views.

Pursue arms control of theater-range nuclear missiles in the region: The United States and Japan should explore nuclear arms control options that can address the security concerns that are driving the pursuit for the SLCM-N. In order to achieve regional-level nuclear arms control and disarmament, it is important to develop collective understanding among regional parties about risks and humanitarian consequences of nuclear use, and promote the idea that taking collective actions will serve their common interests. To this end, the United States and Japan can take the lead by convening a regional conference on humanitarian impacts of nuclear use in Asia, or by exploring their cooperation with regional TPNW supporters such as ASEAN countries to facilitate nuclear disarmament education on these themes.

About the Author

Hideo Asano is Coordinator at the Japan Campaign to Abolish Nuclear Weapons. He coordinates political engagement and policy research activities in Japan. He studied as a Fulbright scholar at the Middlebury Institute of International Studies at Monterey and obtained an MA in Nonproliferation and Terrorism studies. He also earned a BA in International Liberal Arts at Soka University and an MA in Political Science at Kobe University. During his studies at the Middlebury Institute, he was a Graduate Research Assistant at the James Martin Center for Nonproliferation. He also interned at the Global Nuclear Policy Program, with the Nuclear Threat Initiative.
He has attended the Meetings of States Parties to the TPNW as well as the Tenth NPT Review Conference as a Japanese civil society representative. In 2022, he completed the Hiroshima-ICAN Global Academy program.