CHEORWON, South Korea >> To the steady rat-tat-tat of machine guns and exploding bursts of smoke, amphibious tanks slice across a lake not far from the big green mountains that stand along the world’s most heavily armed border.

Dozens of South Korean and U.S. combat engineers build a pontoon bridge to ferry tanks and armored vehicles across the water, all within easy range of North Korean artillery.

For seven decades, the allies have staged annual drills like this recent one to deter aggression from North Korea, whose 1950 surprise invasion of South Korea started a war that has technically yet to end.

The alliance with the United States has allowed South Korea to build a powerful democracy, its citizens confident that Washington would protect them if Pyongyang ever acted on its dream of unifying the Korean Peninsula under its own rule.

Until now.

With dozens of nukes in North Korea’s burgeoning arsenal, repeated threats to launch them at its enemies, and a stream of tests of powerful missiles designed to pinpoint target a U.S. city with a nuclear strike, a growing number of South Koreans are losing faith in America’s vow to back its longtime ally.

The fear is this: That a U.S. president would hesitate to use nuclear weapons to defend the South from a North Korean attack knowing that Pyongyang could kill millions of Americans with atomic retaliation.

Frequent polls show a strong majority of South Koreans — between 70% and 80% in some surveys — support their nation acquiring atomic weapons or urging Washington to bring back the tactical nuclear weapons it removed from the South in the early 1990s.

It reflects a surprising erosion of trust between nations that like to call their alliance an unshakable cornerstone of America’s military presence in the region.

“I think one day they can abandon us and go their own way if that better serves their national interests,” Kim Bang-rak, a 76-year-old security guard in Seoul, said of the United States. “If North Korea bombs us, we should bomb them equally in retaliation, so it would be better for us to have nukes.”

Underscoring those fears: Just hours before the U.S.-South Korean tank drills began in Cheorwon, North Korean leader Kim Jong Un oversaw two ballistic missile test launches meant to simulate “scorched earth” nuclear strikes on South Korean command centers and airfields.

At the heart of South Korean unease is a broader debate over who gets to have nuclear weapons, a question that has anguished nations since two U.S. nuclear bombs flattened Hiroshima and Nagasaki in 1945.

The sharp rise in support for South Korean nuclear weapons is not occurring in a vacuum. Nonproliferation experts say a vibrant global nuclear arms race shows little sign of slowing.

Nine countries — the United States, Russia, Britain, France, China, India, Pakistan, North Korea and Israel — spent nearly $83 billion in 2022 on nuclear weapons, according to a recent report by the International Campaign to Abolish Nuclear Weapons. That’s an increase of $2.5 billion from 2021, with the United States alone spending $43.7 billion.

How South Korea deals with the nuclear question could have major implications for Asia’s future, potentially jeopardizing the U.S.-South Korean alliance and threatening a delicate nuclear balance that has so far kept an uneasy peace in a dangerous region.

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Ironclad. That’s how the United States has long described its commitment to South Korea should a war begin. U.S. officials are adamant that any attack on Seoul by North Korea’s 1.2 million-member military would be met with an overwhelming response.

The United States, bound by treaty to defend Seoul and Tokyo, stations 28,500 troops in South Korea and another 56,000 in Japan. Tens of thousands of Americans live in greater Seoul, a sprawling area of 24 million people about an hour’s drive from the inter-Korean border.

“The ironclad commitment is not just words; it’s a reality. We’ve got thousands of troops right there,” Gen. Mark Milley, who was then the top U.S. military officer, recently told reporters in Tokyo. An attack, said the now-retired Milley, “would spell the end of North Korea.”

Asked about the South Korean public’s support for creating its own nuclear force, Milley said, “The United States would prefer nonproliferation of nuclear weapons. We think they’re inherently dangerous, obviously. And we have extended our nuclear umbrella to both Japan and South Korea.”

South Korean Defense Minister Shin Wonsik said recently that he and his U.S. counterpart signed a document in which Washington agreed to mobilize its full range of military capabilities, including nuclear, to defend the South from a North Korean nuclear attack.

Many in Seoul, however, would prefer nuclear weapons of their own.

North Korea’s only advantage over the South’s high-tech military is nuclear bombs, Kim Taeil, a recent university graduate, said in an interview.

“So if South Korea gets nuclear weapons, we’ll secure an advantageous position where North Korea can’t rival us.”

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While the idea of South Korea pursuing its own nukes has been around for decades, it was rarely mentioned in public by senior government officials. That changed in January when conservative President Yoon Suk Yeol said that his nation could “acquire our own nukes if the situation gets worse.”

“It would not take long,” he said, while also raising the possibility of requesting that the United States reintroduce nuclear weapons into South Korea.

At an April summit in Washington, Yoon and President Joe Biden took steps to address such South Korean worries. The result was the Washington Declaration, in which Seoul pledged to remain in the nuclear Non-Proliferation Treaty as a nonnuclear weapons state, and the United States said it would strengthen consultations on nuclear planning with its ally. It also said it would send more nuclear assets to the Korean Peninsula as a show of force.

Not long after the meeting, the USS Kentucky became the first nuclear-armed U.S. submarine to visit South Korea since the 1980s.

Opponents of South Korea obtaining nuclear weapons said they hope the declaration will reassure a nervous public.

“No one can tell 100% for sure” whether a U.S. president would order nuclear strikes to defend Seoul if it meant the destruction of an American city, Wi Sung-lac, a former South Korean nuclear envoy, said in an interview at his Seoul office.

That’s why the greater consultations called for between the allies in the Washington Declaration are needed to “manage the situation (so) we can tone down public anger and frustration,” he said.

Part of the worry in Seoul can be traced to the presidency of Donald Trump — and to his possible reelection in 2024.

Trump, as president, repeatedly suggested that the alliance, far from “ironclad,” was transactional. Even as he sought closer ties with North Korean leader Kim Jong Un, Trump demanded South Korea pay billions more to keep American troops on its soil and questioned the need for U.S. military exercises with South Korea, calling them “very provocative” and “tremendously expensive.”

“No matter how strong of a security commitment President Biden makes now, if someone who espouses isolationism and an America-first policy becomes the next U.S. president, Biden’s current commitment can become a mere scrap of paper overnight,” Cheong Seong-Chang, an analyst at the private Sejong Institute in South Korea, said in an interview.

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South Korean support for nuclear bombs can also be linked to North Korea’s extraordinary weapons advancements and to the Russian invasion of Ukraine.

North Korea first tested an intercontinental ballistic missile capable of reaching the U.S. mainland in 2017. While the North is still working to overcome technological hurdles with its ICBMs, the weapons have fundamentally changed the region’s security calculus.

One of the poorest countries on Earth, North Korea may now have an arsenal of 60 nuclear weapons and has declared that it is deploying “tactical” missiles along the Korean border, implying its intent to arm them with lower-yield nuclear weapons.

While the Koreas have avoided major conflict since the end of the Korean War in 1953, deadly skirmishes and attacks in recent years have killed dozens.

If future violence escalates, some observers believe that North Korea, outmatched by U.S. and South Korean firepower and fearing for the safety of its leadership, could resort to using a tactical nuclear bomb.

“There is probably no conventional-only scenario in Korea anymore,” according to Robert Kelly, a political science professor at Pusan National University in the South. “North Korea would rapidly lose a conventional conflict. Pyongyang knows this, dramatically raising the likelihood it will use nuclear weapons first, at least tactically.”

Russia’s war against Ukraine may also be showing South Koreans that even friendly nations may hesitate to fully help a country battling a nuclear-armed enemy. Kim’s visit earlier this year to Russia, where he met President Vladimir Putin and toured weapons facilities, has raised fears that North Korea could receive technology that would boost its nuclear program.

“We absolutely need nuclear weapons. Basically, peace can be maintained only when we have equal power to (our enemy’s),” Kim Joung-hyun, a 46-year-old office worker in Seoul, said. “If you look at the Russian-Ukraine war, Ukraine can’t handle the Russian invasion on its own, other than begging for weapons from other countries.”

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Opponents of a nuclear-armed South Korea point out that the strong public support for nukes likely doesn’t calculate the high costs, nor the damage to ties with ally Washington and to crucial trade with neighbor China. Seoul going nuclear could lead to sanctions targeting South Korea’s export-dependent economy.

There’s also concern it could encourage sometimes-rival Tokyo to consider developing its own atomic weapons program.

Some are pushing for a less drastic solution to South Korea’s unique security worries.

“We don’t have other options except inviting American tactical nukes to the Korean Peninsula,” Cheon Seong-whun, a former presidential adviser to a past conservative government, said in an interview. This, he said, would allow South Korea to use those weapons if North Korea uses its tactical nukes, but wouldn’t harm the alliance with Washington.

John Bolton, Trump’s national security adviser from 2018 to 2019, has written that redeploying U.S. tactical nuclear weapons in South Korea would also “buy valuable time for Seoul and Washington to evaluate fully the implications of South Korea becoming a nuclear-weapons state.”

The Washington Declaration and follow-up high-level meetings between the allies have reassured many in Seoul, according to Richard Lawless, a former senior U.S. State Department and Central Intelligence Agency official dealing with nuclear proliferation in Asia.

“The (South Korean) nuclear option genie is not yet back in the bottle, but it is being successfully contained,” he told The Associated Press via email.

Still, Lawless said, there remains “the deeply felt conviction among some senior politicians and among many in the populace” that the only real way to deter nuclear-armed North Korea is for South Korea to have its own nuclear weapons capability. “That concern is now mostly below the waves, but it remains and would be awakened with some passion.”

However the debate ends up, many in Seoul, on all sides of the issue, share another strong conviction.

“There’s a 100% certainty that the North Korean threat will grow,” said Cheon, the former presidential adviser. “North Korea will definitely not stay silent.”