Exploring the Possibility of Nuclear Attacks on Japan
Nuclear Simulations in the Era of "Three-Front Nuclear Threats"

Hiroyasu Akutsu


Today, Japan faces "three-front nuclear threats" from North Korea, Russia, and China. Over the past 30 years following the first nuclear crisis on the Korean Peninsula, North Korea has conducted six nuclear tests, with the sixth test yielding an estimated 160 kilotons. They have developed intermediate-range ballistic missiles targeting Japan and are now capable of launching intercontinental long-range ballistic missiles (ICBMs) powered by solid fuel, which could reach the United States. North Korea is also focusing on developing both strategic and low-yield nuclear weapons. Russia, following its military invasion of Ukraine, not only engages in blatant nuclear blackmail but has also suspended the US-Russia nuclear disarmament process. They have decided to deploy low-yield nuclear weapons in Belarus. China is projected to increase its current nuclear arsenal of over 400 nuclear warheads to 1,500 by 2035. 1 Although the primary target of these three nuclear states is the United States, Japan, being a US ally and host to US military bases, is not exempt from the threat of nuclear attacks. Then, is there a likelihood that Japan could be exposed to nuclear strikes from these three countries?

This is a tricky question, for a nuclear attack against Japan may be possible but not highly probable. While Japan's security environment has become more challenging in recent years, the country has not experienced a nuclear attack since Hiroshima and Nagasaki. The extended deterrence of the Japan-US alliance is considered to remain in effect. 2

Some also argue freeing "possibility" from the numerical constraints of probability theories and simply substituting it with a word "scenario" of everyday language.

This paper will adopt stepwise approaches. First, it will narrow down the conditions under which a nuclear attack on Japan is probable through analysis of the nuclear doctrines and behavior of North Korea, China, and Russia, and then will present possible scenarios. This paper is also intended to present the results of numerical simulations of damage to Japan resulting from a nuclear attack and to raise some issues about the political implications of a nuclear attack on Japan from the perspective of extended deterrence.

To avoid the complexity of discussion, this paper will not detail such cases in which an unforeseen event or accident in a nuclear crisis triggers a nuclear attack or an intentional nuclear attack is made under the guise of an accident.

1. Research Approaches to Assessing the Possibility of Nuclear Attacks

In security studies, academic approaches to the likelihood of a nuclear attack by a state on another state include qualitative and quantitative ones. Among qualitative approaches are the three—now classic—models proposed by Graham T. Allison, focusing on the decision-making process; some other related approaches; 3 and still some that discuss the nuclear posture of a nuclear state to gain an outlook of the future rather than the immediate likelihood of a nuclear attack. 4 Quantitative approaches, on the other hand, include mathematical models, probability theories, 5 Quantitative approaches, on the other hand, include mathematical models, probability theories, 6

In discussing the possibility of the use of nuclear weapons in the ongoing Russian invasion of Ukraine, recent research raises questions regarding the limitations of numerical probability studies and favors understanding "possibility" as a pathway of events rather than relying solely on numbers. 7 The present author proposes substituting the term "possibility" with "scenario" to encourage the exchange, among debaters, of subjective probabilities related to a scenario. Through this interaction, it is hoped that a shared perspective can emerge, leading to a convergence of opinions. 8 Such convergence could serve as a basis for making forecasts.

This paper will adopt qualitative approaches for discussion, taking into account the aim of this SSDP workshop. Yet, since the physical consequences of damage from a nuclear attack can be quantified relatively easily, it will present the outcome of numerical simulations of such damage.

The governments of Russia, China, and North Korea are a dictatorship, and it may be reasonable to assume that "the will of their leaders is the will of the nations." Therefore, in relation to Allison's three models, this paper presupposes rational actors discussed in his first model. Indisputably, in a nuclear crisis, a nuclear attack could occur independently of the intentions of the dictator or the decisions of the highest decision-making body, as in the case of the B-59 incident during the Cuban Missile Crisis. However, such is rather an unforeseen event. 9 This point will be mentioned again later in this paper when discussing simulation scenarios.

2. Rationale for Nuclear Attacks and Conditions for Nuclear Use as Stated in Nuclear Doctrines

The primary strategic target for these three countries is not Japan but the United States. However, this does not guarantee that Japan, a US ally and host to US military bases, will be immune from potential nuclear attacks. Then, under what circumstances could Japan become a target of their nuclear attacks?

In considering the rationale for a nuclear power to unleash a nuclear attack on another country and conditions and context for such an attack, we first seek clues in the attacker's nuclear doctrine, and then examine, as the range of limited conditions for an actual nuclear attack, specific sets of circumstances and political interactions between the states concerned that are assumed to evolve (the exchange of words and communication). The remainder of this section is a brief review of the nuclear doctrines of Russia, China, and North Korea.

2.1 Nuclear Doctrine of Russia 10

According to Russia's nuclear doctrine stated in official Russian military documents, the conditions for Russia to use nuclear weapons can be summarized in the following seven points:

  1. In case weapons of mass destruction, including nuclear weapons, are used against Russia or a Russian ally.
  2. In case the Russian state faces an existential crisis due to an enemy attack by conventional weapons.
  3. The possibility of the preemptive use of nuclear weapons is not ruled out.
  4. In case Russia receives credible information regarding the launch of ballistic missiles to attack the territory of the Russian Federation and/or its allies.
  5. In case an enemy uses nuclear weapons or other weapons of mass destruction against the territory of the Russian Federation and/or its allies.
  6. In case the enemy interferes with government or military installations of the Russian Federation of critical importance, of which dysfunction would thwart retaliatory actions by Russian nuclear forces; or
  7. In case the Russian Federation faces an existential crisis due to aggression using conventional weapons.

In sum, the conditions for the use of nuclear weapons by Russia are cases where its national survival and security interests face an intolerable crisis. In this respect, Russia would likely follow the same logic as China to use nuclear weapons. The same may be true for North Korea, which will be discussed later. A nuclear attack on Japan is hard to discuss only from the perspective of Russia's nuclear doctrine.

Given the current state of Japan-Russia relations, Russia has never directly used nuclear blackmail against Japan or verbally suggested a nuclear strike. But as long as Japan is a US ally, the anxiety on the Japanese side that Russia will use nuclear blackmail against it or suggest a nuclear strike will not be dispelled. A peace treaty has yet to be signed between Japan and Russia; Japan's Northern Territories issue remains untouched. Although bilateral trade relations have been maintained in a limited way since the start of the Russian military invasion of Ukraine, the prospects for improvements in the bilateral relations are obscure.

Russia may have already installed missile facilities on Iturup, and it has recently been reported that Russia is conducting military exercises in anticipation of a "land invasion of the Northern Territories" by other countries. 11 Such behavior of Russia makes the possibility of a Russian nuclear attack on Japan undeniable. It is considered to be in the context associated with Japan's Northern Territories that such an attack becomes probable. In particular, Russia has continued to conduct military exercises around Japan even after the start of its military invasion of Ukraine in February 2022, and has not stopped joint exercises with China. This can be interpreted as Russia wanting to send a signal that it has not forgotten its military considerations for the Far East even during the Ukraine war. Even if Japan has no intention of doing so, Russia might well contemplate, "If we were in Japan's place, we would attempt to retake the Northern Territories while Russia is stuck in Ukraine. To thwart this move, we must strengthen effective control over the Northern Territories and intensify military exercises in the vicinity of Japan." In retrospect, is this precisely the way Russia invaded Japan's Northern Territories after the end of the Pacific War?

2.2 Nuclear Doctrine of China

China's nuclear doctrine is extremely opaque, but at least the following three elements have been pointed out: 12

  1. Minimum nuclear deterrence
  2. No first use
  3. Launch on Warning (LOW)

The specific conditions that would prompt China to use nuclear weapons remain uncertain. However, China's "core interests" provide hints on scenarios in which they might consider such a course of action. The most likely case will be when it judges that an intolerable crisis has arisen, especially over Taiwan—as it declares "the Taiwan question is at the core of China's core interests." 13 In fact, China has expressed conditional willingness to employ force, including nuclear weapons, against Taiwan. China's employment of force against Taiwan may be limited to the following six cases: 14

  1. Taiwan officially declaring independence.
  2. Actions indicating movement towards Taiwanese independence.
  3. Political instability within Taiwan.
  4. Taiwan acquiring nuclear weapons.
  5. Indefinite postponement of unification dialogue between China and Taiwan.
  6. Foreign military intervention in Taiwan's internal affairs.

A nuclear attack on Japan could occur in the case of (6). Yet, the foreign country in China's mind, in that case, is the United States; therefore, it is probable that low-yield nuclear weapons, like those carried by precision missiles such as the DF-26, would be employed against Japan. 15 Japan has repeated public references to Taiwan and has each time been criticized by China for "interfering in its internal affairs." However, China has not resorted to apparent nuclear blackmail against Japan, which indicates low likelihood of a Chinese nuclear attack. Nonetheless, the probability of such an attack could rise under specific political or military circumstances.

China has conducted 45 nuclear tests and is rapidly enhancing its nuclear capabilities. 16 Media reports suggest that Beijing is contemplating a review of its commitment to the no-first-use principle 17 to forestall foreign intervention in a Taiwan emergency. 18 Given China's strong desire to reunify Taiwan, the likelihood of a direct nuclear attack on Japan would increase in the context or scenario of China's invasion of Taiwan.

2.3 Nuclear Doctrine of North Korea (Updated September 2021)

In 2013, North Korea released its first nuclear doctrine with the start of the simultaneous pursuit of nuclear and economic development. Pyongyang updated it in September 2021. According to the updated version, 19

Principles of Using Nuclear Weapons

  1. The DPRK shall regard it as its main principle to use nuclear weapons as the last means in order to cope with outside aggression and attack seriously threatening the security of the country and the people.
  2. The DPRK shall neither threaten non-nuclear weapons states with its nuclear weapons nor use nuclear weapons against them unless they join aggression or attack against the DPRK in collusion with other nuclear weapons states.

Conditions of Using Nuclear Weapons

  1. In case an attack by nuclear weapons or other weapons of mass destruction was launched or drew near is judged.
  2. In case a nuclear or non-nuclear attack by hostile forces on the state leadership and the command organization of the state's nuclear forces was launched or drew near is judged.
  3. In case a fatal military attack against important strategic objects of the state was launched or drew near is judged.
  4. In case the need for operation for preventing the expansion and protraction of a war and taking the initiative in the war in contingency is inevitably raised.
  5. In other case an inevitable situation in which it is compelled to correspond with catastrophic crisis to the existence of the state and safety of the people by only nuclear weapons is created.

North Korea also officially stated that in addition to a nuclear attack in the conventional sense, it intends to generate a tsunami by detonating a "nuclear underwater weapon," thereby destroying enemy ships and military ports. 20

Principle (2) is a limited "Negative Security Assurance (NSA)," which implies that Japan is also a target of a nuclear attack. In addition, while uncertainty remains regarding the current status of North Korea's nuclear warhead development, the language has so far clearly indicated its intention to launch a nuclear attack. 21

3. The "Three-Front Nuclear Threat": Scenarios of Nuclear Attacks on Japan

Then, what scenarios are conceivable, in which these three countries would launch a nuclear attack on Japan? Given the common regime characteristics of these three countries, this paper proposes scenarios in which a nuclear attack on Japan is decided from the perspective of protecting the regime and territorial integrity. However, our scenarios do not detail a storyline consisting of specific events but just represent a context or situation where the possibility of a nuclear attack is likely to increase.

Our scenarios suspend the assumption that no nuclear attack on Japan has so far occurred due to the effectiveness of the US extended deterrence and instead assume a situation in which the effectiveness of deterrence is declining.

In many cases, our scenarios posit a situation where tension escalates due to nuclear blackmail and the incentive to use nuclear weapons reaches a threshold level. 22 The prevailing belief is that nuclear blackmail indicates an impending nuclear attack. However, if the attacker has strong motivations to launch nuclear strikes, would they not have a compelling reason to conceal signs of an imminent attack? This was evident during the Cuban Missile Crisis, where US reconnaissance satellites detected the deployment of Soviet missiles in Cuba. Likewise, the first nuclear crisis on the Korean Peninsula began with the detection of North Korea's nuclear facilities by US reconnaissance satellites. Given the ongoing technological competition between a country willing to launch a nuclear attack and another country that must detect and respond to signs of such an attack at the earliest stage, simulations may assume a situation where advantageous opportunities will arise for the aggressor. 23

3.1 North Korea: Scenarios of Armed Unification and Addressing Imminent US-ROK Military Attacks

As noted above, the countries North Korea regards as its primary enemies are the United States and South Korea; North Korea identifies the United States as a nuclear target. Although not indicated in North Korea's nuclear doctrine, the North Korean constitution articulates "peaceful reunification of the mother country" as the ultimate national goal. That said, if North Korea perceives that the US-ROK alliance is loosened; that US military intervention is unlikely or deterrable, judging that it is capable of dealing with the ROK's counterattack capabilities, the country will probably resort to unilateral armed unification. In the process, North Korea could unleash a nuclear strike on Japan in retaliation to Japanese intervention based on Japan-US alliance cooperation, along with its response to the ROK's counterattack capability and a limited US military intervention.

Another more likely scenario is as follows: When North Korea, whose air defense capabilities are still fragile, unilaterally identifies US/ROK actions, such as joint US-ROK air force exercises, as "imminent armed attacks" or "imminent beheading operation by hostile forces," it will launch a nuclear attack on Japan as an unignorable obstacle as in the case of the armed unification.

The means of delivery would be ICBMs for targeting the US mainland, short-range missiles for attacking the ROK and the USFK, and intermediate-range missiles for striking Japan. For North Korea, the most pressing task is the completion of technology to mount a nuclear warhead on an ICBM, which appears to have not yet been achieved. Therefore, from a probabilistic standpoint, the risk of a nuclear attack on the United States is low at this moment. Yet, this risk will increase in the future.

Incidentally, in connection with North Korea's nuclear attack on the United States, one may recall the fantasy novel The 2020 Commission Report on the North Korean Nuclear Attacks against the United States written by a nonproliferation researcher Jeffrey Lewis, which was published following former US President Donald Trump's "fire and fury" statement. 24 In this novel, North Korea attacks the US mainland, Japan, and the ROK with nuclear-armed ballistic missiles; as a result, the Kim Jong-un regime collapses due to retaliatory attacks by the United States; however, the United States, Japan, and the ROK are also hit by nuclear weapons and suffer massive damage.

Although the novel's realism may be debatable, author Jeffrey Lewis employed the outcome of a scientifically based simulation in describing nuclear damage. 25 Readers may feel that the end of the North Korean regime is realistic. It is unclear, however, what North Koreans think about this endgame or how the novel affects the current North Korean regime.

NUKEMAP, introduced earlier, has come to be used by many researchers; US-based North Korea monitoring site 38North also used NUKEMAP to put out an article on the impact of a fictitious North Korean nuclear attack on Japan and the ROK. 26

3.2 China: Scenarios of Armed Reunification of Taiwan and Invasion of the Senkaku Islands

The situation in which a Chinese nuclear strike on Japan becomes more likely is that Japan is perceived as an obstacle to protecting China's core interests. The first scenario would be about armed unification of Taiwan, and the second about an invasion of the Senkaku Islands.

Since 2022, simulations and wargames featuring or related to the "Taiwan contingency" have been in vogue at home and abroad. The SSDP also hosted intensive discussions on the topic on various occasions. Therefore, I will make just a brief reference to China. Based on my experience, I would like to emphasize a scenario in which the "Taiwan contingency" shifts seamlessly to an invasion of the Senkaku Islands. In such a scenario, China would engage in nuclear blackmail against Japan as part of its message not to meddle in the Taiwan issue. In such cases, if Japan took no action, China would invade the Senkakus unchecked, and things would be too late for Japan. Meanwhile, if Japan attempted to prevent China from invading the islands, China would choose a limited nuclear attack on Japan, or at least a limited nuclear detonation accompanied by no demonstrative attack.

3.3 Russia: Scenarios of Enhancing Effective Control of Japan's Northern Territories

During the Cold War, the greatest threat to Japan was a large-scale invasion by the former Soviet Union. If Russia believes that the Northern Territories, now under its effective control, are threatened by Japan, it will use force, including a nuclear strike.

In 2016, the Russian military deployed a new surface-to-ship missile system to the islands of Iturup and Kunashir. And on December 1, 2020, the Eastern Military District announced deploying the S300V4 surface-to-air missile system to the Kuril Islands (Japan's Northern Territories and Chishima Islands). Reportedly, a more specific deployment destination is Iturup. Russia is bent on military buildup in Japan's Northern Territories under its effective control, indicating readiness to strengthen its air defense posture. According to the announcement, the S300V4 is supposed to protect vital facilities from the threat of ballistic missiles. Although the S300V4's deployment destination has not been disclosed, a Russian Defense Ministry-affiliated media reportedly released a video recording the installation of the system on Iturup. 27

4. Nuclear Attacks and their Consequences

In considering the possibility of a nuclear attack on Japan, it is likewise necessary to consider its consequences if nuclear weapons are used. If the first strike rendered the entire state unsustainable, that would be the end of the matter. Yet, if the state managed to survive the collapse of deterrence, it must consider what steps to take next including measures to protect citizens. In such a case, Japan's response would vary depending on the physical damage caused by the nuclear attack. To visualize potential damage limited to human casualties, the present author attempted simple computer simulations with NUKEMAP as a simulator, 28 summarizing the outcome in Table 1. Incidentally, on March 31, 2023, Nagasaki University's Project on "Reducing the Risk of Nuclear Weapons Use in Northeast Asia (NU-NEA)" released nuclear war simulation results, which, however, fail to include simulation results specific to a nuclear attack on Japan. 29

Table 1. Estimated Damage from Nuclear Attacks on Japan (Surface Burst)

Attacker Target City Yield Injuries Fatalities
North Korea Tokyo 150kt 593,830 246,130
North Korea Yokosuka 150kt 230,700 121,510
China Tokyo 50kt 125,260 289,360
China Yokosuka 50kt 71,460 57,130
Russia Tokyo 50Mt 10,339,790 11,372,360
Russia Yokosuka 50Mt 2,000,290 393,190

(Source: Alex Wallerstein, NUKEMAP, simulated on March 26, 2023.)

In a separate simulation from the simulation that created Table 1, the present author simulated damage caused by a 5-kiloton low-yield nuclear weapon. For example, if the Ministry of Defense in Ichigaya were targeted, the estimated fatalities would be 22,380 and the estimated injuries 47,340. 30 If the Tokyo Metropolitan Government Office were targeted, the estimated fatalities would amount to 40,650, and the estimated injuries to 149,300. 31

As Table 1 illustrates, the scale of human casualties varies depending on the yields of nuclear weapons. When it comes to the United States, the highest decision-making body, including the President, are trained through playing wargames 32 like "Ivy League82" used by the Reagan administration, to examine possible responses for the survival of the presidency and government in the event of a nuclear attack on the country.

More importantly, there is a question of how the US response to a nuclear attack on Japan would change depending on the damage. This would be an important point regarding the extended deterrence, including the credibility of the extended deterrence, such as whether the United States would launch a nuclear attack against an aggressor nation, and if so, with what yield against what target.

5. Red Teaming in Policy Simulations

As mentioned earlier, as examples of policy simulations of the use of nuclear weapons based on historical facts and focused on the decision-making process, we frequently encounter the simulations that simulate the decision-making process of a specific US administration, particularly during events like the Cuban Missile Crisis. 33 This is likely due to the United States' involvement in numerous historical cases of nuclear weapon use or nuclear attacks. 34

Based on the above discussion, the present author proposes a simulation that emphasizes simulating not only the Japanese and American decision-making process regarding hostile nuclear attacks but also the decision-making process in North Korea, China, and Russia regarding the use of nuclear weapons. In this policy simulation, countries acting as adversaries are referred to as the Red Team, and the process of forming a more realistic Red Team is called Red Teaming. 35 When creating a scenario of nuclear attacks against Japan by North Korea, China, and Russia and conducting a simulation based on that scenario, it is necessary to carefully conduct Red Teaming to simulate the highest decision-making processes of these three countries involving their leader.

Of course, there are limitations to conducting a complete simulation of the adversary's internal decision-making process. Multiple factors influence the decision-making process, in addition to the issue of mirror imaging. While this paper has assumed the absolute authority of the North Korean, Chinese, and Russian dictators in conceiving the scenarios, they are not entirely free actors. They are influenced not only by internal interactions but also by external interactions. The workings of such interactions in a simulation can often be unpredictable until actually conducted. The inner workings of closed states like these three countries may be opaque, making Red Teaming more challenging. However, conducting repeated simulations with different participants using various specific scenarios may reveal previously unnoticed aspects such as the types of nuclear weapons and means of delivery, and targets. Furthermore, in scenario development, it is crucial to incorporate contingencies as elements in either the scenario development phase or the simulation implementation phase.


Policy simulations that focus on the decision-making process, along with wargaming that emphasizes the dynamic development of events, will help us understand nuclear tensions as depicted in the scenarios presented in this paper. We, in Japan, should actively utilize simulations and wargaming, learning methodologies at hand that recommend repeating them with minimal time and cost to achieve meaningful results.

Hiroyasu Akutsu is Professor of Heisei International University.

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