CHEORWON, South Korea (AP) - 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 a lake, 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. 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.

North Korea's repeated threats to launch nuclear weapons at its enemies and its tests of missiles designed for pinpoint strikes on U.S. cities have made South Koreans lose faith in America's vow to defend their country.

The fear is that a U.S. president would hesitate to use nuclear weapons to defend South Korea while knowing North Korea could retaliate with a nuclear strike killing millions of Americans.

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.

“I think one day they can abandon us and go their own way if that better serves their national interests,” Kim Bang-rak, a 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.”

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 kept an uneasy peace in a dangerous region.

U.S. officials are adamant any attack on Seoul by North Korea's 1.2 million-member military would be met with an overwhelming response.

Asked about the South Korean public's support for creating its own nuclear force, Gen. Mark Milley, at the time the top U.S. military officer and now retired, 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.”

That's not been enough to quell worry in South Korea.

In January, conservative South Korean President Yoon Suk Yeol broke a longstanding taboo when he said that his nation could “acquire our own nukes if the situation gets worse.”

At an April summit in Washington, Yoon and President Joe Biden agreed on 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.

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 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 who opposes indigenous nukes, said in an interview at his Seoul office.

There's also fear in Seoul about North Korea's extraordinary weapons advancements.

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

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.

“We absolutely need nuclear weapons. Basically, peace can be maintained only when we have equal power to (our enemy's),” Kim Joung-hyun, an 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.”

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.

Some are advocating a less drastic option.

“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.

The Washington Declaration, meanwhile, has 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.

Still, Lawless said via email, 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.”

Correspondent Hyung-jin Kim in Seoul contributed to this report.