President Barack Obama's historic visit to Hiroshima, Japan, on May 27 to honor the memory of those killed and injured when the US dropped the atomic bomb on the city in 1945, comes, ironically, as the argument that Japan should reconsider the nuclear option has reemerged.

But it is not the Japanese who are breaking the "nuclear taboo"; it is a prominent American: the Republican Party’s presumptive nominee for president.

Japan has long eschewed the acquisition of nuclear weapons, choosing instead to rely on the United States to deter any potential threat from its nuclear neighbors. But Donald Trump recently stated that should he become president, he would consider ending the US commitment to Japan’s defense and encouraging it to develop its own nuclear arsenal.

This is a nightmare scenario for Japan. A US decision to abandon its strategic commitment to Japan would create irreversible trauma for the only nation in the world that has experienced the use of these weapons of mass destruction.

But urging Japan to "go nuclear" would change far more than just the US relationship with its longstanding ally; it would also stimulate a cascade of decisions that would profoundly alter the strategic dynamics that have maintained peace in the Asia-Pacific for generations.

It could, in short, become Asia’s nightmare.

Japanese have a deep aversion to nuclear weapons

Tokyo reacted to Trump’s statements with shock, disbelief, and horror. Japanese Foreign Minister Fumio Kishida immediately stated, "It is unthinkable that Japan would possess nuclear weapons in light of its stance of placing emphasis on the Treaty on the Nonproliferation of Nuclear Weapon (NPT) framework."

The Japanese aversion to nuclear weapons comes from experience. Seventy-one years after the end of World War II, Americans and Japanese see the use of the atomic bombs differently. The war came to a cataclysmic end, with many Americans believing the use of nuclear weapons hastened its end and saved American lives. Even today, according to a 2015 Pew Research poll, the majority of Americans think the use of those weapons was justified, although younger Americans are increasingly skeptical.

Japanese, however, take the opposite view: 79 percent believe it was wrong to use the atomic bombs, although a significant minority recognizes the wartime calculus. When it comes to nuclear weapons, few in Japan advocate for the nuclear option.

In fact, Japanese public antipathy to nuclear weapons is so strong and widely known that few have even bothered to examine what the Japanese people think about their country’s security in the nuclear era. Gallup in 1998 explored differences in Japanese and American attitudes about nuclear weapons as the Cold War came to an end: 82 percent of Japanese felt the development of these weapons was bad, while only 61 percent of Americans did. Few Japanese thought that Japan was threatened by a nuclear attack, and a strikingly high number of Japanese (90 percent) thought their country did not need them.

But it’s the US commitment to its defense that’s really kept Japan from going nuclear

Like many US allies in the nuclear era, Tokyo chose to rely on a US guarantee to use its nuclear forces to deter any threats or aggression against Japan — a concept known as "extended nuclear deterrence." This is in contrast to other countries, like France, that opted instead for an independent nuclear capability, even if deployed in a limited arsenal.

But Japan’s postwar leaders have never completely dismissed the nuclear option, and have particularly resisted the notion that the postwar Japanese constitution prevented the exercise of that choice. Over the years, political leaders occasionally let slip the idea that Japan may have to consider the dreaded nuclear option should their country’s security be at stake.

From early on in Japanese parliamentary debate over Article 9 of the postwar constitution, the Japanese government allowed the theoretical possibility that national defense could include nuclear weapons. It would seem that even the country most devastatingly affected by the nuclear revolution still felt the need to hedge its bets, in case the US nuclear guarantee became unavailable.

In fact, through and beyond the Cold War, external events often drove Japan’s security planners to reconsider the nuclear question. The Chinese acquisition of a nuclear arsenal in the 1960s, deliberations over the nuclear nonproliferation treaty (NPT) in the 1970s, and the end of the Cold War in the 1980s prompted policy reviews that revisited Japan’s decision to remain a non-nuclear power.

In each instance, the result was to reinforce the conclusion that the acquisition of nuclear weapons would worsen Japanese security by alarming its neighbors and stimulating a nuclear arms race in Northeast Asia. Japan’s topography made the deployment of ballistic missiles vulnerable to attack, and its proximity to the Asian continent meant it would be almost impossible for Japan to protect that force from attack.

Moreover, lingering memories of Japan’s military expansion across Asia a century ago have made Japan’s neighbors particularly sensitive to dramatic changes in its military capability. Japan’s decision to acquire nuclear weapons would thus trigger others to do the same, ultimately creating greater instability in the regional military balance.

With US forces deployed in Japan, the threat or the use of these destructive weapons against Japan was mitigated, and Japan’s neighbors could feel assured that Japan’s security would not come at the expense of their own.

Japan does have some of the world’s best nuclear scientists and engineers dedicated to the peaceful use of nuclear technology. With few energy resources of its own, nuclear power was viewed as one of the few self-sufficient options for meeting Japan’s energy needs. Into the 21st century, Japanese governments sought to increase the share of nuclear power, with some going so far as to rely on nuclear power for up to 40 percent of their country’s overall energy mix.

The meltdown of Fukushima Daiichi reactors after the earthquake and tsunami of 2011, however, shook Japanese confidence in their country’s management of nuclear power. Today, few Japanese want to expand their nuclear power; most want to decrease their country’s reliance, regardless of the cost.

Japan has also become one of the strongest global advocates of nonproliferation and nuclear security. North Korea’s decision to exit from the NPT in the early 1990s brought the importance of nonproliferation closer to home.

Japan has been a consistent supporter of efforts to persuade Pyongyang to abandon its nuclear ambitions, beginning with its funding for the Korean Peninsula Energy Development Organization (KEDO) in the 1990s, then its participation in six-party talks aimed at building a regional understanding on peace on the peninsula, and today’s efforts to lead in the United Nations Security Council sanctions regime against North Korea.

Withdrawing the US commitment to Japan’s defense could change Japan’s strategic calculus — and profoundly alter the security dynamics of the entire region

The prospect of an American president who would withdraw the longstanding US commitment to Japan’s defenses, urging Tokyo to go it alone, challenges every aspect of Japan’s strategic calculus.

For those in Japan who have chafed against dependency on the United States, Trump’s blithe statement that Japan would ultimately end up a nuclear power anyway seems to validate their argument that the United States would betray Japan in the end.

The debate in Japan would not necessarily lead to the decision to acquire nuclear weapons; it is equally possible that the US retreat from its commitment to Japan would open up a debate over a variety of choices, including an alliance with another nation in Asia, a neutral or unaligned state, or a reorganization of Japan’s military for autonomous defense. Popular opinion would not naturally shift to support for the nuclear option.

Whatever choices are debated within Japan, other nations in Asia would be highly sensitive to a Japan untethered from the United States. A kaleidoscopic series of changes in attitudes in Seoul, Beijing, and Tokyo would result. It is too difficult to consider all possibilities, but South Koreans would be particularly anxious about Tokyo’s choices.

Should the US simultaneously end its longstanding alliances with both Japan and South Korea, as Trump has suggested, Northeast Asia would be transformed. Popular sentiments in both Japan and South Korea have become very sensitive to each other, and a potent cycle of reactive nationalism could result.

Seoul’s choices would feed into the Japanese debate. Seoul might see acquiring nuclear weapons as its only path forward for coping with a nuclear North Korea. Or it could seek greater accommodation with Beijing. A South Korea under the protection of China would strengthen Japanese calls for an autonomous military capability, including discussion of a nuclear option if Tokyo planners felt the Seoul-Beijing links were antagonistic to Japan.

Greater strategic cooperation between Beijing and Tokyo could also result, with a condominium of China, South Korea, and Japan emerging to challenge US interests in the Pacific.

Whether nuclear proliferation would result, or whether America’s non-nuclear allies would seek nuclear protection elsewhere, Asia’s geostrategic balance would undergo tremendous change.

The Japanese people may not change their opinion on nuclear weapons, and their leaders may resist the impulse to acquire them. But Japan’s neighbors will likely hedge on the possibility that Japan will return to its status as a fully autonomous military power.

With the US no longer extending its nuclear umbrella and China increasing its military power, the dynamics of Asia’s military balance will become increasingly edgy and susceptible to miscalculation and confrontation.

Sheila A. Smith is a senior fellow for Japan studies at the Council on Foreign Relations and the author of Intimate Rivals: Japanese Domestic Politics and a Rising China (New York: Columbia University Press, 2015). She can be reached on Twitter @SheilaSmithCFR and on Facebook.