by George Friedman
February 7, 2018
Unless the U.S. military does something to stop it (and maybe even then), North Korea is going to become a full-fledged nuclear power, a fact that is stressing the U.S. alliance structure in Northeast Asia. Whether the U.S. decides to attack or learn to live with it, its decision could undermine America’s credibility with its two stalwart partners in the region, Japan and South Korea. And with North Korea on the path to being able to strike the U.S. mainland, the question is looming larger whether the U.S. can be trusted to respond to an attack on Japan or South Korea even when doing so puts U.S. cities at risk.
The U.S. alliance structure could evolve a number of ways over the coming decades. But when examining the degree to which Japan and South Korea will continue to put their faith in U.S. security guarantees – or whether each country could fully break from the U.S. alliance – there’s one inescapable variable: the willingness of either country to continue forsaking nuclear weapons in a region that’s about to be swimming with them. Currently, both countries rely fully on the U.S. nuclear umbrella. Neither wants to find itself holding a knife in a nuke fight.
To develop and sustain a nuclear deterrent, a country needs five things. The first is the capability to build a nuclear weapon. This includes both technical expertise and the ability to procure fissile material. The second is the military capability to ensure that it can deliver a nuclear weapon in what is likely to be an unpredictable combat environment. Overcoming the first two obstacles is exceedingly expensive, meaning the country also needs the economic resources to invest in the nuclear program and accompanying delivery systems in perpetuity. Fourth, it needs sufficient public backing, given that decisions to “go nuclear” often generate major domestic concern about the economic and moral costs of the program, the very real risk of accident and the potential that the effort could backfire and put the country at risk of annihilation. Finally, the decision must be rooted in strategic rationale strong enough to outweigh the risks of international blowback, which can range from isolation and economic sanctions to attempts to halt the nuclear program by force.
This Deep Dive does not seek to make a firm forecast on whether either Japan or South Korea will pursue nuclear weapons. Rather, in an attempt to gauge a key component of the resilience of the U.S.-led security framework in the Western Pacific, it focuses on how well-equipped both Japan and South Korea are to overcome these five hurdles should they be compelled to do so. It concludes that both countries almost certainly have the technical and scientific capabilities to develop a bomb quickly, possibly in less than a year – though building out a robust arsenal, plus the weapons infrastructure needed to ensure delivery, would take longer, particularly for Japan. As sophisticated, stable and wealthy economies, both also could sustain the budgets needed to do so. Public support is a bigger hurdle in Japan than in South Korea. And the strategic rationale would depend on the size of the breach with the U.S. and whether they determined that the status quo had become fundamentally untenable.
As could be expected given its history, nuclear weapons are an extremely sensitive issue in Japan. Partially as a result, Japan has not made any serious effort to develop its own nuclear arsenal since becoming the only country ever to be attacked with a nuclear weapon at the end of World War II. Instead, the alliance with the U.S. has been the cornerstone of Japanese strategic doctrine, with the country sitting firmly beneath the U.S. nuclear umbrella.
Today, the U.S. does not store nuclear weapons anywhere on Japanese soil, but Japan is well-covered by all components of the U.S. “nuclear triad” – where nuclear devices can be delivered from submarines, land-based silos and stealth aircraft – stationed far from Japanese shores. Any leg of the triad could conduct a retaliatory strike on Japan’s behalf following a nuclear attack launched by any adversary in the region.
Japanese leaders have been known to float the possibility of going nuclear during periods of heightened regional tension, but Japan has generally been content to depend on the U.S. for nuclear deterrence, along with most conventional military needs. Doing so enabled Japan to focus resources on rebuilding the country after World War II and on forging a dominant, high-tech economy. Nonetheless, Japan almost certainly could develop and deploy its own nuclear arsenal if it felt the need to do so, possibly within just a few years. This is why Japan is sometimes referred to as a paranuclear, nuclear threshold or nuclear latency state.
Building the Bomb
Japan’s latent ability to develop a nuke is rooted in the fact that it has a highly advanced economy, a burgeoning arms manufacturing base and a well-earned reputation for pioneering the cutting edge of scientific and technical advancement. Most important, Japan – an archipelagic nation with scant energy resources of its own – has had little choice but to build out a sophisticated civilian nuclear power system. Before the 2011 Fukushima disaster, Japan had 54 nuclear power reactors in operation, the third most in the world behind only the U.S. and France. (Following the tsunami, everything was taken offline, but 11 were back in operation by the end of last year, and 12 more have been approved to restart.) In 2012, Japan updated its Atomic Energy Basic Law to describe its civilian nuclear power program as indispensable to its national security, a move that hints at Tokyo’s desire to preserve its nuclear latency.
One byproduct of the emphasis on nuclear power is that Japan is the only non-nuclear weapons state with a large stockpile of separated plutonium. To reduce its dependence on imported enriched uranium, Japan has been focusing on “closing” the nuclear fuel cycle, which involves reusing plutonium extracted from spent fuel. Perhaps most alarming to its neighbors, this separated plutonium can be near-weapons grade. (The U.S. proved in the 1960s that reactor-grade plutonium could be weaponized.) As of 2010, according to the International Atomic Energy Agency, Japan had 10 tons of separated plutonium stored domestically and another 37 tons in France and the United Kingdom (where it is sent to be reprocessed), and it plans to open its own long-delayed reprocessing site this year, even though none of Japan’s nuclear reactors currently online are capable of using reprocessed plutonium as fuel. The Nonproliferation Policy Education Center, or NPEC, says Japan’s stockpiles are already enough to build as many as 6,000 nuclear warheads.
To what degree Japan has the technical expertise to make the leap to nuclear weapons is unclear. Nonetheless, most potential adversaries in the region are under the assumption that Japan already has the expertise to go nuclear quickly. It’s been widely claimed in the media that Japan could do so within six months in a crisis, but there isn’t much publicly available evidence supporting this figure. At best, six months may be sufficient for Japan to produce a crude prototype device, but it would be unprecedented to build out the necessary infrastructure, conduct tests, implement safety and training systems and develop at least one viable delivery system so quickly. More credible estimates range from one to even 10 years, underscoring the high number of factors that could influence the timeline. For context, once they had committed to it, both the U.S. and the Soviet Union achieved nuclear status in about four years.
Delivering the Bomb
Japan wouldn’t necessarily need to develop delivery systems with the scale and diversity to rival the largest nuclear powers. In fact, most smaller nuclear powers have sought to avoid getting locked in a Cold War-style arms race. Still, to achieve minimal deterrence – in which the goal is simply to ensure the ability to retaliate against an attacker – Japan over time would likely try to develop its own nuclear triad.
Owing to constitutional constraints on the development of offensive warfare capabilities, Japan does not have any ballistic or cruise missiles, nor any stealth bombers. The Japan Maritime Self-Defense Force, however, has developed some of the world’s most sophisticated submarine technologies. It also has a long history of successful undersea operations dating back to the Cold War, when Japanese submarines played a vital and by all accounts successful role in preventing Soviet counterparts in the Sea of Japan from threatening U.S. positions in the region.
That this is an area of Japanese strength is convenient, since submarines are the most survivable of the three legs of the nuclear triad, and thus would be where Japan may want to focus most. With Japan’s adversaries just a short distance away, land-based missile sites and bomber squadrons would be particularly vulnerable to getting wiped out quickly in a surprise attack.
Though Japan does not have any submarine-launched ballistic or long-range cruise missiles in its arsenal, its Soryu-class submarines are equipped with harpoon anti-ship missiles that, in theory, could be modified to carry a nuclear warhead, as Israel is believed to have done. Regardless, this would be at best a short-term solution. The Soryu-class diesel-electric attack submarines are ideal for defensive combat operations in the crowded confines of the Sea of Japan and East China Sea, and their air-independent propulsion systems enable them to stay submerged for weeks at a time. But the Soryu subs would likely need to be re-engineered to be capable of launching ballistic missiles or longer-range cruise missiles. The range of harpoons is limited to around 80 miles (130 kilometers), and Japan would want to have larger missiles capable of delivering a multiple independently targetable re-entry vehicle (basically a payload carrying multiple warheads that can hit several distinct targets). In addition, nuclear deterrence is greatly improved the longer a submarine can remain submerged. This may compel Japan to start the long-term project of developing its own class of nuclear-powered submarines, which can stay underwater far longer than their diesel-electric counterparts.
To complete the nuclear triad, Japan would need to make substantial leaps forward in both land-based missile and stealth bomber technologies. Neither is likely beyond Japan’s technical capabilities. But because it’s a narrow and dense island nation without much strategic depth, Japan wouldn’t want to rely too much on land-based missiles, which would be among the first targets in a full-blown war. Japan does not have anything akin to North Dakota, home to a heavy concentration of U.S. nuclear-tipped intercontinental ballistic missile silos that are far from major population centers and safe from both earthquakes and sea-based attacks. Nonetheless, it would want to maintain at least a modest arsenal of land-based ballistic missiles, in part because they can respond the quickest among the triad legs. Submarines must contend with communications delays, while aircraft simply take longer than missiles to reach distant targets.
Though Japan does not yet have ballistic or cruise missiles, it is believed to have much of the technology needed to develop them – or even to develop ICBMs, thanks to its experience with satellite launches. Most of Japan’s likely target sets would be within range of shorter-range ballistic missiles, but Tokyo may also want to be able to hit population centers in western Russia and inland China, which would necessitate an ICBM.
Compared to land- and sea-based missiles, aircraft delivery systems would give Tokyo the most flexibility in designing counterstrike scenarios, since they can be redirected to new targets or recalled if the decision to strike is canceled altogether. They can also more easily attack multiple targets. Aircraft can also be used to deter conflict simply by demonstrating their reach while stopping short of attacking (as illustrated by the frequency of recent U.S. B-1 flights over the Korean Peninsula). Perhaps most important, Japan’s burgeoning fleet of F-15s and, especially, the stealthier F-35A (the first of which was deployed late last month) can be modified to carry tactical nukes, potentially giving Japan some breathing room until its missile program starts to bear fruit. Notably, Japan purchased its first aerial refueling tanker in 2008, improving both the range of Japanese warplanes and their ability to avoid being targeted by enemy surface-to-air missiles. And in December, Japanese officials confirmed that Tokyo is planning to equip its fleet of F-35A warplanes with U.S. long-range cruise missiles capable of striking targets as far as 560 miles away – say, ballistic missile launch sites in North Korea – in what would be Tokyo’s first major purchase of offensive weaponry in more than half a century.
Carrying the Cost Burden
It’s hard to gauge the exact costs of a nuclear program, given that spending levels among existing nuclear powers are closely guarded secrets. Costs would also vary widely depending on the scope of the nuclear deterrent Japan chose to pursue, as well as whether it could modify existing weapons systems for nuclear use or would need to develop entirely new systems. Naturally, credible estimates have varied widely as well.
An unconfirmed report in 2006 by the newspaper Sankei, purportedly citing government estimates, claimed that developing Japan’s first prototype would cost about $1.68 billion to $2.52 billion over 3-5 years. According to the NPEC, sustaining a full-fledged nuclear program would cost Japan from as low as $7 billion to as high as $400 billion per decade, depending on the size of the nuclear arsenal and accompanying delivery systems. For context, Japan will spend some $48 billion on defense in 2018 out of an overall government budget of around $860 billion.
Since 1976, Japan has capped military spending at 1 percent of GDP. In 2017, this was still enough to give Japan the world’s eighth-largest military budget, according to the Stockholm International Peace Research Institute. But to sustain at least minimal deterrence, Japan would need to spend far more on its military than it has been for most of the past half century. With the world’s third-largest economy, and one that has proved capable of sustaining enormous levels of spending, Japan almost certainly has the resources to do so if it were deemed a priority. North Korea and Pakistan (albeit apples and oranges compared to Japan) have evidently been successful with less. Notably, last year, Japanese Prime Minister Shinzo Abe formally discarded the 1 percent cap on military spending.
Perhaps the biggest hurdle facing Japan is political will. As the only country with any experience getting attacked by a nuclear weapon, Japan has a deep-rooted aversion to nukes. Open advocacy for their development is typically met with public scorn. Following World War II, Japan’s “nuclear allergy” even extended to nuclear power – and unease has remained to the point that, after the Fukushima disaster, the government in 2011 moved to phase out the use of nuclear power altogether by 2040, though this policy was reversed just a year later.
Poll results vary, but they don’t generally suggest that the public is clamoring to go nuclear. A string of surveys in September, for example, showed around 80-90 percent in opposition – even though the polls took place just days after North Korea’s sixth nuclear test and little more than a month after its first successful ICBM test.
Japan would also need to shed major legal constraints to go nuclear, such as its Atomic Energy Basic Law, which bans all but peaceful nuclear activities. (Japan’s “three non-nuclear principles” – that Japan shall neither possess nor manufacture nuclear weapons, nor shall it permit their introduction into Japanese territory – has never formally been made law.) Tokyo is also a signatory of the Treaty on the Non-Proliferation of Nuclear Weapons and the Comprehensive Nuclear Test-Ban Treaty. Withdrawal from those treaties could jeopardize supplies of enriched uranium that Japan would need until it succeeded in closing the fuel cycle.
Potentially most problematic is Article 9 of Japan’s pacifist constitution, which forbids the use of military force for offensive purposes. Thus, any weapons acquired or developed by the military must be deemed intended for defensive purposes. Abe said in 2016 that nuclear weapons wouldn’t violate Article 9 if used only in defensive situations – and several politicians have since echoed this sentiment – but it remains to be seen whether this would survive a legal challenge or the court of public opinion. At a minimum, Article 9 is restricting Japan’s ability to develop or procure the delivery systems needed for an assured second-strike capability.
Laws, of course, can evolve. Japan has reinterpreted elements of its constitution repeatedly since it first rebuilt its military. Abe’s administration approved a reinterpretation of Article 9 in 2014 to grant the military powers to exercise the right of collective self-defense. These changes were formalized through a pair of contentious security laws implemented in 2016. And Japan has been gradually edging into the realm of offensive warfare capabilities anyway. Still, Abe has made it a priority in 2018 to fully amend Article 9 – and his party has the supermajority in the legislature to see the changes through. But such a revision would also need to pass a public referendum. Here, polls are less than favorable, underscoring just how politically contentious Japan’s path to remilitarization is likely to be even if it stops short of nuclear weapons.
It’s hard to gauge how the Japanese public would react to a serious push to go nuclear, in part because such a move would take place only in an environment in which it had become abundantly clear that Japan could not rely on the U.S. for its nuclear deterrent. In such an environment, the public view could be different. Japan, after all, has shown an extraordinary ability to reinvent itself throughout history.
Unlike Japan, South Korea has had relatively few qualms about going nuclear, and it likely already would be a nuclear power if it weren’t for the United States. Seoul quietly launched a full-fledged nuclear program in 1971 after the withdrawal of some 26,000 U.S. troops from the country. But the program was still in the early stages when it was shut down in 1975 under pressure from Washington. After the Jimmy Carter administration tried to withdraw from the peninsula completely, Seoul tried again to purchase a reprocessing facility from France, and again it was stopped by Washington. Until 2010, the U.S. – which didn’t want to get dragged into a war – also imposed tight limits on the range of South Korean missiles and the weight of their payloads.
Instead, Washington persuaded Seoul to rely on the hundreds of nuclear weapons the U.S. already had deployed on South Korean soil, which peaked at more than 900 in the 1960s. Following the end of the Cold War in 1991, with long-range strike options having negated the need for deployment on the doorstep of potential adversaries ringing the Korean Peninsula, the U.S. withdrew the entire arsenal.
South Korea has taken advantage of this umbrella to build out one of the world’s most advanced economies and to invest in stout conventional military forces. Still, the South has never felt fully at ease being tied to the United States. And today, it has the capabilities, resources and strategic rationale to classify as a latent nuclear state. Moreover, unlike Japan, the South already has some of the weapons systems in place to achieve minimal deterrence, and it faces much lower domestic resistance to the development of nuclear weapons.
Building the Bomb
South Korea’s ability to rapidly go nuclear is likewise rooted in its civilian nuclear infrastructure. The country has 24 nuclear reactors, with another eight slated to come online before 2029. Among these are four pressurized heavy water reactors, which produce near-weapons-grade plutonium as waste. (President Moon Jae-in is pushing plans to phase out nuclear power altogether by 2060, but it’s doubtful at this point that this policy will survive.)
South Korea imports all the enriched uranium it uses for civilian purposes. There isn’t much available to be mined in the country. And it does not have any known industrial-scale uranium enrichment facilities, which would be needed to have an independent source of weapons-grade highly enriched uranium – and which would be difficult to keep secret. This means South Korea would likely look first to plutonium, which can be obtained by reprocessing spent nuclear fuel. According to a 2015 NPEC report, South Korea already has enough plutonium to produce 4,330 relatively low-yield nukes.
There’s also considerable evidence that South Korea already has the technical expertise to transition from civilian to military nuclear activities. Though the South’s nuclear program shut down in 1975, Seoul admitted in 2004 that scientists with the Korea Atomic Energy Research Institute had been quietly continuing to experiment with fissile material ever since (purportedly without the government’s knowledge), in violation of its international treaty obligations. According to the IAEA, the scientists had demonstrated the capability to extract plutonium from spent fuel (albeit only in small amounts) and enrich uranium up to 77 percent – shy of the 90 percent threshold considered weapons-grade but still theoretically sufficient for a nuclear weapon. Estimates of South Korea’s “breakout time” are similar to Japan’s – and similarly inexact. But a crude prototype is likely well within reach.
It’s important to note that South Korea does not have much of its own uranium, and thus it relies on nuclear fuel provided by members of the Nuclear Suppliers Group, which requires buyers to be signatories of the Treaty on the Non-Proliferation of Nuclear Weapons. If South Korea were to withdraw from the treaty, sourcing nuclear fuel would not become impossible, but it would certainly be made more difficult.
Delivering the Bomb
Like Japan, South Korea would need to consider the type and scale of the nuclear arsenal it wants to build to determine the weapons systems it would need. But unlike Japan, South Korea already has much of the delivery systems it would need to achieve minimum credible deterrence. Going nuclear would, however, change the emphasis somewhat of the country’s military modernization efforts.
For example, South Korea’s rapidly advancing arsenal of Hyunmoo ballistic and cruise missiles, with ranges of 200-950 miles, are believed to be capable of carrying a nuclear warhead. And this arsenal is set to get bigger. Last fall, the U.S. removed restrictions that had capped the range of South Korean missiles at roughly 500 miles and their payload to about 1,100 pounds, freeing the South to develop missiles that are better suited for nuclear purposes. The Hyunmoo-3D supersonic cruise missile in development is expected to have a maximum range of almost 2,000 miles, which would put all but the outer reaches of China within reach. Seoul unveiled plans last year to develop the Hyunmoo-4 ballistic missile, or Frankenmissile, capable of carrying a 1-2 ton nuclear payload. Given South Korea’s proximity to most high-value targets in North Korea, northeastern China and Tokyo, it may not need to develop an ICBM, though it would if it wanted to put all of China and Moscow in range. The South’s recent successes launching a satellite into orbit suggest it has attained much of the necessary technology to produce an ICBM.
Perhaps most notable considering that South Korea is a peninsular country surrounded by potentially hostile maritime powers, the South is also developing a submarine-launched variant of the Hyunmoo-II. This underscores another key element of the South’s preparedness to go nuclear: its fleet of advanced submarines and deep experience with undersea operations. Most important of this fleet is the Jangbogo-III class of submarines currently under construction. They are expected to be able to carry 10 Hyunmoo-3C cruise missiles and an unknown number of submarine-launched ballistic missiles. Last fall, Moon and U.S. President Donald Trump reportedly discussed the South buying or jointly developing nuclear-powered submarines from the U.S. The main advantage of nuclear-powered subs over their faster, quieter diesel counterparts is that they can stay underwater for much longer — a valuable feature in nuclear deterrence.
South Korea does not have any strategic bombers, but it does have a number of fighters, including a growing fleet of F-15s, F-16s and F-35s, that could be fitted to deliver small nukes akin to the U.S. B-61, which boasts a variable yield of 0.3 to 340 kilotons. (The bomb used in Hiroshima had a yield of around 15 kilotons.) The South also has an advanced aviation industry and likely has the capability to develop strategic bombers over time. The South also has begun acquiring aerial refueling aircraft.
Carrying the Cost Burden
Once again, accurate estimates on potential nuclear spending are elusive. South Korea may be able to spend less than Japan, given its existing missile and submarine assets, but the total cost burden would again depend on an array of factors.
What is clear is that South Korea has the world’s 11th-largest economy, with a 2017 GDP of $1.4 trillion. And it has the world’s 10th-largest military budget. Unlike Japan, South Korea has routinely spent far more than 1 percent of its GDP on its military, including 2.7 percent last year. In a scenario in which the U.S. exits the peninsula completely, there would be a high number of competing budgetary priorities. Nonetheless, it’s safe to assume the South could carry the cost burden of going nuclear if doing so became a priority.
But there may be heavy ancillary economic costs to going nuclear if doing so leads to international sanctions or jeopardizes trade relationships. Moreover, as noted above, withdrawing from the Non-Proliferation Treaty would jeopardize the South’s supply of uranium, almost certainly harming its civilian nuclear power sector and forcing the country to either seek higher-cost fuel sources or bite the bullet and switch away from nuclear energy. It would also undermine South Korea’s plans to become a major exporter of civilian nuclear technologies at a time when it’s been gaining a foothold in Middle Eastern markets.
South Korea does not have an overwhelming aversion to the development of nuclear weapons. Polls vary, with public support generally increasing during times of greater tension with North Korea. But a Gallup poll from September found that around 60 percent of South Koreans support going nuclear — a figure that’s roughly in line with the historical trend, even though the poll was taken shortly after North Korea’s sixth nuclear test.
Accordingly, the question of whether to pursue nuclear weapons is a regular topic of public debate. Conservative political parties — which usually dominate South Korean politics – and ideologically aligned media routinely make the public case for becoming a full-fledged nuclear state or, at minimum, redeploying U.S. tactical nukes on the peninsula. Once in office, South Korean leaders have generally been much more circumspect about this prospect, but likely not because of overwhelming political pressure from opponents of going nuclear.
Strategic Risks and Rationale
Both Japan and South Korea are trapped in something of a strategic paradox. They do not have nuclear weapons, but because of their latent breakout ability and their tight alliance with the U.S., their neighbors are behaving in many ways as if they do.
Both countries are investing heavily in stiffening their defenses against a possible attack, particularly through sophisticated anti-ballistic missile systems. Japan is primarily procuring sea- and land-based U.S. systems such as Aegis, some of them jointly developed. South Korea, in comparison, is focusing on indigenous land-based systems, while also allowing the U.S. to expand deployments of advanced systems such as Terminal High Altitude Area Defense. But even the most advanced ballistic missile defense systems are still far too unreliable to build a strategic doctrine around. South Korea has also invested heavily in missiles capable of striking, say, a North Korean launch site if a nuclear attack were deemed imminent – and Japan appears primed to follow suit. But these too are unlikely to fully eliminate the threat posed by North Korea. They certainly would not mitigate the much more expansive Chinese nuclear capabilities. In short, both Japan and South Korea need nuclear deterrence. And for now, that means they need the United States.
But to whatever degree Japan and South Korea believe that the U.S. would respond on their behalf if attacked, they cannot be fully confident that the U.S. would be willing to put its own cities at risk to do so – particularly as adversaries develop increasingly long-range strike capabilities. Moreover, as the current Korean crisis illustrated, both countries – but Japan, in particular – have little freedom of action when it comes to shaping regional events. Japan could not, for example, launch an attack on North Korea unilaterally if the U.S. opposed it, nor could it use the threat of such an attack to gain leverage at the negotiating table. We think South Korea has dissuaded the U.S. from going to war with the North, but it realizes it would not be able to stop the U.S. from launching military operations in the North if it resolved to do so, despite the risk of a devastating counterattack on Seoul.
So it’s not hard to imagine scenarios in which core interests between the U.S. and either of its two stalwart allies in Northeast Asia diverge to the point where Japan or South Korea decides U.S. guarantees are no longer enough. There doesn’t even need to be a full break in the alliance for this to happen (as the U.K. and France demonstrated when they went nuclear in the 1950s and 1960s despite being members of NATO), and there are plausible scenarios in which the U.S. would even support such a move. Japan and South Korea’s other option is to opt for an Israeli-style nuclear doctrine of “strategic ambiguity,” where the country’s nuclear program reaches fruition but is never declared publicly.
Still, capability is only part of the strategic calculus, and the risks of going nuclear for either country shouldn’t be underestimated. If either Japan or South Korea developed nuclear weapons, it would almost certainly escalate the regional arms race (starting with the other of the two following suit), raising the risk of accident and miscalculation while eroding nonproliferation efforts globally. The domestic political backlash, particularly in Japan, could be immense and destabilizing, to say nothing of the international diplomatic and economic repercussions. If it weren’t already at that point, the diplomatic approach to denuclearizing North Korea would be finished. Meanwhile, going nuclear might also weaken U.S. attention to the needs of either country. Even if South Korea and Japan no longer needed the U.S. nuclear umbrella and were less tied to U.S. direct interests, the U.S. would likely remain incomparably powerful and uniquely handy in a crisis.
For either to go nuclear would mark a profound departure from a status quo that has, in most ways, enormously benefited both South Korea and Japan. Any such decision would presumably be done only reluctantly, and in recognition that the status quo is fundamentally no longer tenable.