SEOUL, South Korea — As the United States debates the wisdom of military action against North Korea, its allies in South Korea have largely moved on and reached an uncomfortable conclusion — that they may have no choice but to live with a nuclear-armed neighbor.

U.S. and South Korean forces began twice-yearly war games today aimed at preparing for a possible attack by the North. But the idea of trying to knock out the North’s nuclear arsenal with a pre-emptive strike is a nonstarter across the political spectrum in South Korea, where millions live in range of North Korean artillery and rockets.

“In South Korea, whether you are a conservative or progressive, military action is not an option you can choose,” said Koh Yu-hwan, a professor of North Korean studies at Dongguk University in Seoul, the capital. “It’s an option of mass destruction.”

That broad consensus was behind President Moon Jae-in’s assurances to South Korea last week that he stood firmly against a military strike on the North and that the Trump administration would seek the South’s consent before any such action. On Monday, Moon emphasized that the joint exercises this week were defensive in nature.

But if South Korea’s leaders have essentially ruled out the “preventive war” that the Trump administration says it is considering, they are still grappling with what options that leaves them.

Moon contends that North Korea can be persuaded to forsake nuclear weapons through a combination of deft diplomacy and tough economic sanctions. Yet many across South Korea find that prospect highly unlikely. Ask people in the South whether the North will ever abandon its arsenal, and more often than not, the answer is: Would you if you were North Korean?

Still, South Korean politicians cannot openly agree with those like Susan E. Rice, a national security adviser to President Barack Obama, who recently argued that the world could tolerate and contain North Korea in the same way it did the nuclear threats from the Soviet Union and China during the Cold War.

They cannot do so not because they do not believe that — many do — but because it would be politically difficult to accept the North’s nuclear arsenal without putting South Korea on equal footing with its own nuclear weapons.

“Even if the North has effectively become a nuclear power, acknowledging it as one without South Korea itself going nuclear is politically untenable,” said Paik Hak-soon, a senior analyst at the Sejong Institute, a think tank south of Seoul.

At the same time, South Korean leaders are unwilling to accept the view of Rice’s successor, Lt. Gen. H.R. McMaster, who has maintained that “classical deterrence theory” cannot be applied to the North’s leader, Kim Jong Un, given his unpredictable nature and brutal record. Some South Korean analysts fear that such thinking would all but justify starting a war.

It has not been lost on South Korea that Washington seemed to recognize the urgency of North Korea’s nuclear threat only after it demonstrated an ability to hit the United States with intercontinental ballistic missiles last month — and not after earlier intelligence assessments concluded that the North could mount a nuclear warhead on a short-range missile capable of hitting the South.

To South Koreans, the idea that North Korea would fire a nuclear-armed ICBM at the United States without being attacked is absurd. They argue that Kim knows the United States would retaliate by destroying the North and that they do not regard him as suicidal. Instead, the fear in South Korea is that the North will try to use its nuclear arsenal to drive a wedge between the United States and its allies in the region.

In some ways, it is already doing so.

The more President Donald Trump and his advisers speak of attacking North Korea in a “preventive war,” the more South Koreans worry that the United States is putting its security ahead of their own. A recent interview in which Sen. Lindsey Graham said that Trump believed that “if thousands die, they’re going to die over there — they’re not going to die here” has exacerbated such suspicions and generated widespread outrage.

At the same time, a common question among South Koreans these days is whether the United States will hesitate to come to their aid after the North demonstrates it can deliver nuclear warheads to U.S. cities.

(While its missiles appear capable of striking the West Coast and much of the Midwest, it is unclear whether the North has mastered the technology required for a warhead to survive the heat and pressure of re-entering the atmosphere.)

In recent years, the United States has repeatedly sent nuclear-capable strategic bombers over South Korea to signal that its “nuclear umbrella” extends over the nation. But some local newspapers have dismissed the flights, likening them to “police patrol cars that pass through the neighborhood but never catch the thief.”

Kim Sung-han, a former vice foreign minister who is a professor at Korea University in Seoul, said the United States and South Korea must discuss new ways to assure the public of the United States’ commitment to protect it.

With public confidence in the United States slipping, recent polls show a majority in South Korea favor arming the country with nuclear weapons of its own. And last week, the main conservative opposition group, the Liberty Korea Party, formally called on Washington to reintroduce tactical nuclear weapons that were withdrawn from South Korea in the early 1990s. The proposal, though popular, presents the South Korean government with another dilemma: Officials said the presence of nuclear arms in the country would make it far more difficult to persuade the North to give up its own.

Confusion over the Trump administration’s strategy has only contributed to anxiety in South Korea.

In the past two decades, Washington has repeatedly tightened sanctions against North Korea. But “the more they isolated and pressured the North,” said Yang Moo-jin, a professor at the University of North Korean Studies in Seoul, “the more advanced its nuclear program has become.”

Moon has urged a more flexible approach, suggesting that Washington’s past insistence that North Korea commit to nuclear disarmament as a precondition for talks to begin may no longer be realistic. Instead, he has argued that the priority should be a freeze of the North’s nuclear and missile programs, though he has not been specific about how to achieve that.

“The solution to the North Korean nuclear issue has to start with a nuclear freeze,” he said during a nationally televised speech last week. “We don’t want the North to collapse. We will neither pursue unification by absorbing the North nor seek artificial unification.”

Koh, the Dongguk University professor, said the government’s best hope might be to transform North Korea in the long term “like in the old Soviet Union” by encouraging economic liberalization and promoting the flow of information into the isolated nation, including South Korean movies and television dramas.

But others analysts warn that may be more wishful thinking. Unlike China and Russia, they note, North Korea shares a peninsula with a neighbor with which its people will always draw direct comparisons. As long as the South remains the more successful Korean state — its economy is now more than 20 times bigger — the autocratic government in North will consider it an existential threat, they argue.

“A nuclear North Korea will not behave like a conventional nuclear state,” said Lee Sung-yoon, a Korea expert at the Fletcher School of Law and Diplomacy at Tufts University. “Its nuclear threats will only grow.”

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