| Introduction

On June 24, 2026, the Liberal Democratic Party (LDP) and Japan Innovation Party (Nippon Ishin no Kai, JIP), the two parties making up Japan's ruling coalition, each submitted policy proposals to Prime Minister Takaichi Sanae (高市早苗)¹⁾ on the planned revision, by year-end, of Japan's security-related documents (hereafter the "three security documents").²⁾ The submission of these proposals is far more than a mere statement of intra-party opinion; it marks the political starting point that indicates the direction in which the Japanese government will restructure the three documents, and functions as a de facto benchmark for coordination. The three security documents, Japan's National Security Strategy (NSS), National Defense Strategy (NDS), and Defense Buildup Program, are the core texts that define the basic framework for Japan's defense objectives, its approach to deterrence, budget formulation, and the operation of its alliance. The proposals can accordingly be read as a signal of how far Japan's security policy is likely to expand, and at what speed and intensity.

The revision of the three security documents commands attention because Japanese security policy is shifting from a traditional defensive concept, the exclusively defense-oriented policy (senshu boei), toward a technology-centered concept of layered deterrence. As asymmetric and hybrid battlefield elements such as drones, artificial intelligence (AI), cyber, space, and electronic warfare continue to expand, Japan has concluded that the Self-Defense Forces' conventional capabilities alone are insufficient to meet future threats. As a result, strengthening defense capacity is being redefined: rather than simply adding troops or acquiring equipment, it now extends to the industrial production base, intelligence-collection systems, technological self-reliance, and the capacity to sustain combat operations (継戦能力). The proposals are documents that reveal what level of military autonomy and deterrent capability Japan intends to pursue within this broader push to strengthen its defenses.

Notably, although the LDP and JIP are coalition partners, they diverge clearly on the pace and scope of security reform. The LDP places its weight on structural, incremental change, whereas JIP has called for a more radical security overhaul through concrete numerical targets and a reexamination of long-standing defense taboos. This suggests that the government's revision of the three security documents will involve not a single technical edit, but an unavoidable political process of reconciling strategic priorities within the ruling coalition. This article examines how the differences between the LDP and JIP proposals could shape the actual direction of the revision, and analyzes, step by step, what repercussions those policy choices could have for security on the Korean Peninsula and for Korea-Japan relations. Its focus, moreover, lies less in the intra-party policy differences themselves than in how those differences are reconciled once the proposals enter the actual policymaking process.

| The Significance of the Policy Proposals and the Process for Revising the Three Security Documents

The policy proposals submitted by the LDP and JIP in June 2026 function, in effect, as the government's basic guidelines for the full-scale revision of the three security documents it is about to undertake. Under Japan's parliamentary cabinet system, even a document drafted directly by the government can hardly be implemented in practice without the ruling parties' approval and their cooperation on budget formulation and legislation. Proposals worked out in the parties' security research councils and policy coordination bodies thus give the government a political benchmark: if it drafts an original text within the scope of the proposals, party approval can be secured. In other words, a proposal is not simply a statement of opinion; it is a device that sets the coordinates within which the government can act, establishing both the ceiling and the floor for actual policy decisions.

Proposals from the ruling parties and outside experts matter for two main reasons. The first is the formation of political consensus. Since the government can hardly pursue large-scale defense policy without ruling-party approval, the proposals lay the groundwork for the government's original draft. The second is securing a broader range of perspectives and legitimacy. Because contemporary security spans not only military power but also economic security, the protection of advanced technology, information warfare, diplomatic capacity, and supply chain stability, proposals from experts and the ruling parties broaden the scope of policy and strengthen its social legitimacy. In this sense, the June proposals can be seen as both a precursor to the government's own draft and a channel for politically legitimizing future security policy.

Submission is not the end of the process for a coalition proposal; it is subsequently given concrete form as an official government document through the following steps. First, the proposals worked out through intra-party discussion, together with the report of the government's Council of Experts on the Fundamental Reinforcement of Defense Capabilities (防衛力の抜本的強化に関する有識者会議​), submitted on September 19, 2025, are delivered directly to the prime minister. On receiving them, the prime minister orders a government-wide review and sets the direction for policy. The National Security Secretariat then takes the lead in coordinating with the Ministry of Defense, the Ministry of Foreign Affairs, the Ministry of Finance, and other relevant agencies on how to reflect the proposals in actual policy and budgets, drafting the government's original version of the three security documents. At this stage, unrealistic demands are adjusted, and funding options are discussed alongside policy priorities.

The completed government draft is then presented back to the ruling parties for a final round of coordination. Councils within the LDP and the JIP, as well as each party's policy divisions, conduct a rigorous prior review of whether the submitted proposals have been adequately reflected. Where agreement cannot be reached, the document must be revised. Once all internal party procedures are complete, the government formally adopts the newly revised three security documents at a Cabinet meeting. This decision is typically made at year-end, in December, when budget formulation is in full swing. The proposals, then, are not only the starting point for policy design but the key connecting link that drives the entire process through which national policy is ultimately finalized.

| The LDP Proposal: Content and Key Features

The most distinctive feature of the LDP proposal is that it conceives of Japan's security not as the sheer sum of military power but as a combination of six elements of national strength. Diplomatic capacity, defense capacity, economic capacity, technological capacity, intelligence capacity, and human capital are treated as a single integrated security asset, on the premise that the nation's continuous defense is possible only when all of these elements are strengthened simultaneously.³⁾ This reflects a shift in Japan's conception of security beyond the traditional framework centered on troops and equipment, toward a system encompassing technology, industry, and information. The prominence given to drones, AI, and cyber response capabilities is directly tied to the recognition that the outcome of future warfare will be determined preemptively not only on the physical battlefield but in the technological and informational domains as well.

In this context, the LDP proposal defines the security environment Japan faces as an era of "new warfare," and in response emphasizes structural transformation through such concepts as "new methods of warfare (新しい戦い方)" and "securing the capacity to sustain combat (継戦能力の確保)". Underlying this is the perception that threats have become a constant condition, combining not only traditional interstate warfare but nonmilitary means such as gray zone conflict, cyberattacks, competition in the space domain, and pressure related to economic security. The LDP accordingly presents, as a core task, not simple force buildup but the construction of an integrated response system capable of addressing complex threats that blur the boundary between peacetime and contingency. As an attempt to organically combine defense policy with industrial and science and technology policy, this can be assessed as an approach that qualitatively expands the scope of Japan's security strategy.

On the matter of increasing the defense budget as well, the LDP emphasizes structural efficiency and sustainability over the total amount itself. Its position is that resources should be concentrated in areas that secure actual combat sustainability, rather than simply expanding the size of the budget. Specific items presented as core priorities include expanding munitions stockpiles, maintaining and reinforcing defense industry production lines, managing the life cycle of defense equipment, and building a stable supply network. This signals a shift in policy focus from "possessing" military power to "maintaining it in a continuously operable state." In other words, the LDP places policy priority on building long term sustained combat capacity, that is, a "sustainable force," rather than on short term force buildup, a priority closely connected with the recent effort by the Japanese government to reorganize its munitions production base at the national level.

The LDP proposal also cites the need to strengthen "counter strike capability (反撃能力)” as an important element related to the security environment on the Korean Peninsula. This reflects the recognition that, as the advancement of North Korea's nuclear and missile capabilities poses a direct threat to Japan's security, missile defense systems alone no longer provide sufficient deterrence. Japan is accordingly reinforcing the logic that it must possess a counter strike capability capable of deterring or neutralizing an attack once clear signs of it emerge. Political and legal controversy remains over its relationship to the exclusively defense oriented principle, but in practice this is likely to lead to an expanded division of roles under the U.S. Japan alliance and to a shift in operational concepts.

The LDP has made clear its intention to bring about a qualitative transformation of defense capability within a short period. Japan is signaling policy resolve to produce tangible results in the near term, rather than continuing to defer its response to future battlefields as a medium to long term task alone. In particular, the stability of defense equipment production and the ability to rapidly surge production in an emergency are recognized as issues directly tied to economic security, not merely to military strengthening in the narrow sense. Building on this recognition, the LDP proposal reorganizes defense policy and foregrounds structural tasks, technological innovation, maintenance of the industrial base, supply chain stabilization, and sustained combat capacity, as core elements of security.

Ultimately, the LDP's policy proposal can be seen as aiming for a shift away from a security concept centered on traditional military power, toward a whole of nation security system suited to the environment of "new warfare." In seeking to build capabilities and structures that would actually function in a real crisis, rather than a formal buildup of defense capacity alone, it presents an important direction for Japan's security policy grounded in realism.

| The JIP Policy Proposal: Content and Key Features

The proposal of the Japan Innovation Party is characterized by an approach far more direct and numerically oriented than that of the LDP, one that foregrounds the strengthening of deterrence. Most conspicuous is its attempt to reconstruct security around DIME (Diplomacy, Information, Military, Economy), that is, to combine diplomacy, intelligence, military power, and economic strength into a single strategic framework.⁴⁾ This reflects a realistic recognition that military power alone cannot guarantee security, and that diplomacy, intelligence, and the economy must be operated in an integrated manner. Even so, while premising this integrated framework, JIP distinguishes itself from the LDP by urging far more radical decisions with respect to policy instruments.

On the defense budget, JIP emphasizes that "where the money is spent" matters more than "what percentage is spent," while at the same time arguing that Japan's fiscal year 2026 defense budget should be raised immediately to 2 percent or more of GDP, with a medium to long term target of 3 percent or more.⁵⁾ This represents an attempt to raise the baseline of Japan's defense policy above the traditional level of deterrence. Because the figures are explicit, friction with fiscal burdens, revenue adjustments, and welfare spending is unavoidable, yet JIP prioritizes the effectiveness of deterrence over such political costs. In this respect, it can be said to adopt a stronger logic linking fiscal policy and security than the LDP does. In particular, a steady expansion of the defense budget appears intended to raise, over the long term, both operational readiness and the capacity to sustain operations.

A more sensitive area concerns the review of the Three Non-Nuclear Principles and the introduction of nuclear-powered submarines (hereafter "nuclear submarines"). The LDP proposal made no mention of either the Three Non-Nuclear Principles or nuclear submarines.⁶⁾ JIP, by contrast, raised the possibility of a realistic review of the third of the Three Non-Nuclear Principles, the provision against "not permitting the introduction," and also proposed the early introduction of nuclear submarines equipped with vertical launching systems (VLS).⁷⁾ This can be interpreted as challenging one of the symbolic taboos of Japanese security policy, with the intent of lowering the threshold for nuclear deterrence while reinforcing long range, long endurance operational capability. A VLS-equipped nuclear submarine is not simply a matter of platforms; it carries significant implications for Japan's maritime deterrence, combined operations, and the division of roles within the U.S.-Japan alliance. This does not, of course, translate directly into Japanese nuclear armament, but neighboring countries can hardly avoid becoming acutely conscious of Japan's strategic autonomy and its potential for military advancement. Ultimately, the JIP policy proposal is one that seeks to transform Japan's defense policy from defensive minimalism toward a more offense oriented deterrence posture.

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| Outlook and Implications for South Korea

The LDP places importance on phased adjustment and institutional stability, whereas JIP favors a fast-track approach through numerical targets and the reexamination of institutional taboos. This difference is likely to become particularly pronounced with respect to budget expansion, the scope of counter strike capability, the review of the Three Non-Nuclear Principles, and the introduction of nuclear submarines. The forthcoming revision process is accordingly likely to constitute not simple policy consultation but a political negotiation process within the ruling coalition over the permissible scope of security expansion.

The future trajectory of Japan's security policy will likely be shaped by how the gap between "political acceptability" and "strategic necessity" is reconciled. On the review of the Three Non-Nuclear Principles, the likelihood of institutional revision materializing in the near term remains limited, notwithstanding JIP having raised the issue. This is because the anti-nuclear identity rooted in the experiences of Hiroshima and Nagasaki, together with the constraints of public opinion, continue to exert strong influence. In particular, although the principle against "introduction" bears directly on the U.S.-Japan alliance, the domestic political cost of revising it is considerable, making it likely that, for the time being, the matter will remain at the level of maintaining "strategic ambiguity" rather than resulting in formal policy change.

The LDP, together with JIP, has established the introduction of VLS-equipped nuclear-powered submarines as a core agenda item in security policy. The coalition agreement concluded in October 2025 explicitly states an intention to pursue the acquisition of VLS-equipped submarines featuring "next-generation propulsion systems," a provision interpreted as having effectively formalized the introduction of nuclear submarines. The Takaichi government regards this as part of a defense buildup in which "no option is excluded," and Defense Minister Koizumi Shinjiro (小泉進次郎) has likewise maintained the position that, amid a changing security environment, both conventional submarines and nuclear propulsion should be examined.

Even so, caution coexists within the LDP and among parts of the political establishment. It has been pointed out that, given the enormous budget required to operate nuclear submarines and the accompanying need to secure specialized personnel and resolve technical challenges, substantial practical barriers remain to actual policy implementation. In particular, because the Three Non-Nuclear Principles carry strong symbolic weight in Japanese society, concerns persist regarding the political burden and backlash that could result from unsettling them. For this reason, the LDP has proceeded cautiously, officially employing the expression "review of next-generation propulsion systems."

VLS-equipped nuclear submarines thus remain a contentious issue within Japanese politics, and the likelihood of their introduction will be determined less by technical necessity than by political acceptability with respect to the Three Non-Nuclear Principles, the exclusively defense-oriented principle, and the operational concept of the Maritime Self-Defense Force. That said, JIP's recent public raising of the issue suggests that, compared to the past, this agenda item has entered the realm of policy review rather than remaining peripheral. In this connection, caution prevails within the LDP, caught between the need to strengthen strategic deterrence and the burden of domestic public opinion, such that the likelihood of immediate institutionalization remains limited. Nevertheless, the fact that discussion of introducing VLS-equipped nuclear submarines has arisen reflects the recognition that such submarines could substantively strengthen maritime deterrence, on the basis of extended submerged endurance and stealth, without leading directly to nuclear possession in the manner of future nuclear armament. In particular, given China's naval buildup and the changing operational environment in the East China Sea and Western Pacific, VLS-equipped nuclear submarines could function, from Japan's perspective, as "a means of enabling a strategic leap while maintaining the non-nuclear principles."

In this process, the Korea variable may also serve as an important point of reference. Should South Korea sustain defense spending at 3 percent of GDP or higher, or move further toward the full-scale introduction of nuclear submarines, this could further underscore, within Japan as well, the need to respond to a changing strategic environment. Indeed, the fact that the LDP proposal references South Korea's defense spending ratio, and that the JIP proposal cites South Korea's introduction of nuclear submarines, indicates that Japan regards shifts in the military capabilities of regional allies and partner countries as an important point of comparison. It is accordingly difficult to rule out the possibility of a pattern of "convergent competition," in which military buildups between South Korea and Japan come to be mutually referenced.

In this light, the revision of Japan's three security documents is not merely an adjustment of Japan's internal defense policy; in that it may have direct repercussions for the security environment on the Korean Peninsula and for the broader military order of Northeast Asia, it calls for a preemptive response on South Korea's part.

First, the establishment of a ROK-Japan consultative mechanism regarding the expansion of Japan's counter strike capability is urgently needed. The more Japan strengthens its long-range strike capability and integrated deterrence beyond its own national defense, the more directly the manner in which these capabilities are operated could affect South Korea's security and operational environment. An institutional consultative structure is accordingly needed, one that encompasses the scope of intelligence sharing, criteria for target designation, conditions for the initiation of operations, and advance notification procedures aimed at preventing accidental clashes. This matters not merely as a form of friendly cooperation, but as a minimum safeguard for reducing the possibility of military misperception and unintended conflict between the two countries. In particular, given that the moment counter strike capability is actually exercised, the repercussions could affect both alliance cooperation and regional stability, the establishment of advance control mechanisms is of considerable importance.

Second, national-level preparation is needed for the possibility of a regional arms race and a nuclear domino effect. Developments surrounding Japan's increase in defense spending and strengthening of deterrence could trigger responses from North Korea and China, with the result that Northeast Asia as a whole risks falling into a vicious cycle of mutual buildup. In this process, the Korean Peninsula could serve as the most sensitive zone of impact, and South Korea's security burden would inevitably grow as well. South Korea should accordingly interpret Japan's military buildup not as an isolated matter but within the structure of regional arms competition. The key lies in managing Japan's shift so that it does not invite miscalculation by neighboring countries, while simultaneously strengthening South Korea's own deterrence and crisis response capabilities.

Third, South Korea should strengthen its own defense capability and strategic autonomy, while managing this not as simple symmetrical competition with Japan's military buildup, but rather in a mutually referential manner amid changes in the regional security environment. That is, the purpose of strengthening South Korea's defense capability is not to provoke Japan, but to enhance South Korea's crisis response capability in preparation for the security uncertainty that could result from Japan's shift. In this respect, the military buildups of the two countries need to be understood not solely as adversarial competition, but as a complex competitive structure in which mutual observation and institutional management proceed in parallel.

The most reliable way to respond to changes in Japan's security policy is to reduce dependence on the external environment and to raise South Korea's own capacity for crisis response. Alongside strengthening military deterrence, South Korea must simultaneously pursue the institutionalization of ROK-U.S.-Japan cooperation, mechanisms to prevent accidental clashes between Korea and Japan, the non-proliferation management of historical and territorial disputes, and the strategic stabilization of relations with China and Russia. In particular, given that changes in Japan's security policy could serve as a catalyst for ROK-U.S.-Japan cooperation while simultaneously carrying the risk of reigniting ROK-Japan historical and territorial disputes, multilayered management is required. South Korea's response must accordingly encompass not only military buildup but also the strengthening of diplomatic coordination capacity and crisis management systems.

1. 自由民主党, “新たな国家安全保障戦略等の策定に向けた提言,” 2026年6月9日 <https://storage2.jimin.jp/pdf/news/policy/213453_1.pdf> (Accessed: June 30, 2026); 日本維新の会, “「危機の30年」時代の国家安全保障戦略,” 2026年6月17日 <https://o-ishin.jp/news/2026/images/e72e644b904e5e593444322ca613dee901afb7ea.pdf> (Accessed: June 30, 2026).
2. Japan's Ministry of Defense officially uses the designation "Three Strategic Documents," but the term "three security documents" is the one in general use, including in media coverage. The term used throughout this article follows this common usage.
3. 自由民主党, “新たな国家安全保障戦略等の策定に向けた提言,” p.7.
4. 日本維新の会, “「危機の30年」時代の国家安全保障戦略,” pp.5-6.
5. 日本維新の会, “「危機の30年」時代の国家安全保障戦略,” pp.71-72.
6. Maehara Seiji (前原誠司), president of the Japan Innovation Party's Research Commission on Security, emphasized two points in submitting the party's proposal to Prime Minister Takaichi. The first was that Japan's nuclear strategy be clearly settled: that is, while maintaining the principles of "not producing" and "not possessing" nuclear weapons under the Three Non-Nuclear Principles, he hoped a realistic review would be conducted of the principle of "not permitting introduction." The second concerned the VLS-equipped, next-generation-propulsion submarine, a matter also addressed in the coalition agreement. 日本維新の会, “提言【「危機の30年」時代の国家安全保障戦略】を高市総理に提出しました,” 2026年6月24日 <https://o-ishin.jp/news/2026/06/24/18420.html> (Accessed: June 30, 2026).
7. 日本維新の会, “「危機の30年」時代の国家安全保障戦略,” pp.65-67; pp.41-42.

※ The opinions expressed in 'Sejong Focus' are those of the author and do not represent the official views of Sejong Institute.