On October 24, 2020, Hondulas deposited its instrument of ratification of the Treaty on the Prohibition of Nuclear Weapons (TPNW), bringing the number of countries that have ratified the treaty to over 50. In accordance with Article 15, the Treaty will thus enter into force 90 days later, that is, on January 22, 2021. The TPNW, which was adopted at the UN General Asembly in July 2017 with the approval of 122 countries, is the first treaty on nuclear weapons to prohibit its States Parties to (a) develop, test, produce, manufacture, acquire, possess, or stockpile, (b) transfer, (c) receive, (d) use or threaten to use, (e) assist, encourage, or induce anyone to engage in any activity prohibited to a State Party, (f) seek or receive any assistance from anyone, to engage in any activities prohibited to a State Party, and (g) allow any stationing, installation, or deployment of nuclear weapons (Article 1).

Considering the enthusiasm of the proponent countries and NGOs, including the International Campaign for the Abolition of Nuclear Weapons (ICAN), at the time of the adoption of the TPNW, it looks to have taken some time for the Treaty to come into effect. In addition, at the time of writing, the signatories were limited to 84 countries. Furthermore, Sweden and Switzerland, both of which voted in favor of its adoption, have stated their intentions not to sign the Treaty at present. Nevertheless, given the challenging situation surrounding nuclear disarmament, including the demise of the Intermediate-Range Nuclear Forces (INF) Treaty and the difficulty of extending the New Strategic Arms Reduction Treaty (New START), proponents of the TPNW are exploring ways of using this opportunity of its entry into force to increase the number of States Parties and reinvigorate international and domestic public opinion.

For the proponents, one of the immediate and important targets is the signing of the TPNW by Japan and other non-nuclear-weapon states under extended nuclear deterrence (or nuclear umbrellas) provided by allied nuclear-armed states. Signing of the TPNW by those states under nuclear umbrella—which, along with the nuclear-armed states, did not participate in the UN conference to negotiate the TPNW (except the Netherlands), and have consistently made it clear that they will not join the treaty—would mean a significant change in their policy of relying for their national security on nuclear deterrence and acceptance of a norm of the prohibition of nuclear weapons. Proponents of the Treaty expect that it will further strengthen international pressure on the nuclear-armed states to abolish nuclear weapons. In September 2020, 56 former presidents, prime ministers, foreign ministers and defence ministers from non-nuclear-weapon states allied with the United States as well as former secretaries-general of NATO signed an open letter organized by the ICAN calling for participation in the TPNW.¹

Japan has had the experience of making a political decision to join, in response to growing international and domestic public opinion, the Convention on the Prohibition of Anti-Personnel Mines (Ottawa Convention) and the Convention on Cluster Munitions (Oslo Convention), both of which codify norms for the prohibition of certain weapons on a humanitarian basis. During their negotiations, Japan, which has a long coastline, initially opposed these conventions because it regarded anti-personnel land mines and cluster munitions as important weapons for national security in effectively thwarting adversaries' invasions. However, Tokyo eventually decided to change its policies and accepted the conventions in the face of the rapid development and acceptance of norms for prohibition of those weapons globally as well as increasing international and domestic calls for a total ban on such "inhumane" weapons that caused a number of civilian casualities during and even after civil wars and regional conflicts in various regions, especially Africa and Asia, in the post-Cold War era.

However, at least in the foreseeable future, it is unlikely that Japan will make the same political decision to participate in the TPNW as it did regarding the Ottawa and Oslo conventions. The most important reason for Japan not to join the Treaty is that such a decision would be inconsistent with one of the foundations of its security policy: "with regard to the threat of nuclear weapons, the extended deterrence of the U.S. with nuclear deterrence at its core is indispensable. In order to maintain and enhance the credibility of the extended deterrence, Japan will work closely with the U.S., and take appropriate measures," as stated in its 2015 National Security Strategy (NSS). Once a country accedes to the TPNW, seeking or receiving the provision of extended nuclear deterrence constitutes non-compliance with the Treaty under its Article 1 which prohibits "seek[ing] or receiv[ing] any assistance...from anyone to engage in any activities prohibited to a State Party," as seen above.

Proponents of the TPNW argue that, even if Japan joins the TPNW, it will be able to continue its alliance with the United States and receive extended "conventional" deterrence by renouncing its reliance on nuclear deterrence. However, given the security environment surrounding Japan, it is not possible for Japan to make a decision on such a major change in its security policy at least at this time. Firstly, Japan is surrounded by nuclear-armed North Korea, China and Russia. With great-power and geopolitical competition intensifying amidst a power transition, the security environment in Northeast Asia has been becoming more unstable and uncertain, and these nuclear-armed states attach greater importance to nuclear deterrence. Under these circumstances, even if Japan were to join the TPNW, it is unlikely that those nuclear-armed states would reduce or remove their nuclear threat to Japan, let alone eliminate their nuclear arsenals, as long as security disputes remain with Japan. Secondly, there is a significant difference in national security importance and impact between extended nuclear deterrence on one hand, and anti-personnel landmines and cluster munitions on the other, for which alternatives could be considered. It would be difficult to establish an effective non-nuclear deterrent or regional security system to replace extended nuclear deterrence against the nuclear threat facing Japan. Thirdly, if Tokyo were to renounce its reliance on extended nuclear deterrence, its adversaries might step up their provocations against Japan without regard for the US nuclear deterrent. Fourthly, in a "nuclearized" Northeast Asia, should the United States be asked to defend Japan with conventional forces alone, the United States might even reconsider its alliance with Japan because it could suffer a serious disadvantage to its own forces, including in response to nuclear threats.

Needless to say, Japan, which experienced the atomic bombings of Hiroshima and Nagasaki, is one of the countries that most keenly recognizes the humanitarian consequences of nuclear weapons. Furthermore, in the latter half of the 20th century, particularly since the end of the Cold War, there has been a growing demand for the legal prohibition of certain weapons in a way that squarely addresses the humanitarian dimensions that have become increasingly accepted in the international community. The Ottawa and Oslo conventions are prime examples. Beatrice Fihn argues that "'humanitarian impact' is not a catchy phrase to use in order to create more pressure on nuclear-armed states to pursue disarmament...Instead, it is a reflection of the emergence of a new era in security policy in which humanitarian concerns and humanitarian law are growing in importance."²

However, humanitarianism alone was not the reason for the successful conclusion of the Ottawa and Oslo conventions. The global convergence and acceptance of the norms on banning anti-personnel land mines and cluster munitions also seem to have resulted from the following factors: the relatively low security importance and military effectiveness of the targeted weapons; the possibility of substitution by other weapons; and the existence of countries that do not join or violate the legal prohibition (for instance, military powers such as the United States, China, and Russia that have not acceded to the Ottawa and Oslo conventions) not necessarily having a significant impact on the national security of States Parties.

Historically, the establishment of international norms has been achieved in many cases only to the extent that they do not erode the fundametal national security or undermine the international order. Promotion of nuclear disarmament from a humanitarian approach also needs to be underpinned by a dimension of national security. These circumstances do not pertain to nuclear weapons, at least at the present time. It has also been argued that nuclear weapons have contributed to maintaining the international order since World War II, especially the prevention of a recurrence of a great-power war, precisely because of the enormous consequences that could be expected should nuclear weapons be used. On the other hand, there is still no guarantee that the international order can be maintained in a world without nuclear weapons. Therefore, a fundamental change in perception of the importance of nuclear weapons in national security among nuclear-armed states and nuclear umbrella states that would enable them to accept the TPNW is not likely to occur, at least under the current circumstances. This also implies that global establishment and acceptance of a prohibition norm on nuclear weapons will not be achieved in one leap.

These arguments do not mean that nuclear armed-states and their allies, including Japan, should continue to pursue their nuclear policies without considering the humanitarian dimensions of nuclear weapons or the development of a norm on their prohibition. It is an undeniable fact that the actual use of nuclear weapons is highly likely to cause inhumane devastation. In addition, significant progress in nuclear arms control and disarmament, which would include the total elimination of nuclear weapons in its scope, will require not only an improved security environment that greatly reduces the need for nuclear weapons, but also global acceptance of the norm on prohibition of nuclear weapons. Furthermore, it should be noted that the TPNW was established against a backdrop of deep disappointment and growing dissatisfaction with the situation in which nuclear-armed states have increased their dependence on nuclear deterrence, as nuclear arms control and disarmament have stagnated and regressed since the conclusion of the US-Russia New START in April 2010.³

Opponents of the TPNW have raised concerns that the TPNW may undermine the effectiveness of the NPT in the future. While proponents have repeatedly rejected such a possibility, some experts argue that if nuclear disarmament does not progress under the NPT, then it should be abandoned altogether in favor of the TPNW.⁴ Perhaps this is a "pick-off throw" to press the nuclear-armed states on nuclear disarmament. Still, if this becomes a reality, the foundation of the near-universalized NPT—which comprises the three pillars of nuclear nonproliferation, nuclear disarmament, and the peaceful use of nuclear energy, is a cornerstone of the nuclear nonproliferation regime, and is the only treaty under which five nuclear-weapon states have made a commitment to nuclear disarmament—will be seriously damaged. In this regard, it should be emphasized that the significance of the NPT in nuclear disarmament will not decline despite the establishment of the TPNW.

At the same time, the prime responsibility for reinvigorating nuclear arms control and disarmament, and for taking concrete actions to that end, rests with countries that rely on nuclear deterrence for their national security. Therefore, while carefully seeking an appropriate balance between deterrence and arms control, and between security and norms, those countries need to identify norms and concrete measures on nuclear weapons underpinning security that can be agreed upon by the nuclear-armed states, their allies, and other non-nuclear-weapons states, and to make efforts for expanding them even incrementally.

To this end, discussions between proponents and opponents of the TPNW are essential. The establishment of the TPNW has widened the rift over nuclear arms control and disarmament between those two camps, but at the same time, it has also stimulated debate on these issues. In this context, it is not necessarily clear whether the TPNW meeting of the States Parties—the first meeting is to be held within one year after the treaty's entry into force—as proposed by the proponents would be an appropriate venue for such discussions.⁵ Even if the nuclear-armed states and their allies, including Japan, are invited as observers, there remain several uncertainties at this point in time, including what the States Parties want from the observers' participation, what opportunities will be available for observers to speak and discuss, and whether the meeting will lead to constructive discussions between the States Parties and observers.

On the other hand, there are also other opportunities for discussion. For instance, the "Eminent Persons Group for Substantial Progress in Nuclear Disarmament" hosted by Japan and the "Creating an Environment for Nuclear Disarmament (CEND)" initiated by the United States, marked the beginning of this trend. In addition, the postponed NPT Review Conference is to be held in 2021, bringing together the non-nuclear-weapon states in favor of TPNW and the five nuclear-weapon states and their allies. Considering the current security environment, it is hard to be optimistic that discussions there will lead to immediate outcomes. However, as an essential first step toward the reinvigoration of nuclear arms control and disarmament, serious debate between proponents and opponents of the TPNW in that venue is imperative.