SEOUL — When Ukraine gave up its nuclear weapons in the 1990s, experts debated whether the decision would make the country safer or more vulnerable to an invasion from Moscow, its nuclear-armed neighbor.

Now, as Russia pounds Ukrainian cities while being accused of committing atrocities against civilians, many in South Korea say there is no more room for debate.

Since the conflict began, South Koreans have flooded online chat rooms with discussions about their country’s need to have nuclear weapons to prevent an invasion from North Korea, their own nuclear-armed neighbor. On Tuesday, North Korea warned that it would use its nuclear weapons “at the outset of war,” should one with the South ever start.

Like Ukraine, South Korea once had nuclear weapons within its borders. And Seoul abandoned its own covert nuclear program in the 1970s in exchange for security guarantees from the United States. But as they watch Ukrainians battle Russian forces and plead for outside military assistance, many South Koreans fear that was a mistake.

“There is no justice in this world, only national interests,” said one commentator on Twitter. “We must build our own defense, arming ourselves with nuclear weapons, unless we want to find ourselves in the sorry state Ukraine is in now.”

South Koreans have demanded nuclear weapons for years as North Korea expanded its arsenal and provoked Washington with missile tests. In one recent survey of South Koreans, 71 percent of the respondents supported arming the country with nuclear weapons, according to a research paper published in February by the Carnegie Endowment and the Chicago Council on Global Affairs.

While North and South Korea see the war in Ukraine differently — with the North supporting Russia and the South condemning Moscow’s military adventurism — both countries appear to have drawn similar conclusions from the conflict.

For South Koreans, the war has shown the extent to which a nuclear-armed power can get away with invading a non-nuclear neighbor when fears of nuclear war make intervention less likely. And for the North, it offered further proof of the advantages of a homegrown nuclear deterrent.

Analysts say North Korea is now more determined than ever to keep its nuclear arsenal, as the South confronts its own vulnerability.

“The war in Ukraine is a chilling reminder that when things get really dicey, there is a limit to how much your friends can do for you,” said Cho Kyong-hwan, a member of the Presidential Commission on Policy Planning in Seoul. “At the end of the day, you only have your own power to defend yourself.”

Drawing parallels between South Korea and Ukraine can be misleading. South Korea ranks sixth in the world in military strength and North Korea is 30th, according to the Global Firepower Index, which ranks conventional war-making capabilities. (Ukraine is 22nd and Russia is second.)

Ukraine is not a NATO member and does not have a formal alliance with the United States, whereas Seoul and Washington are bound by a mutual defense treaty.

When the defense chiefs of the United States and South Korea held their annual meeting in December, Washington renewed its commitment to “extended deterrence,” vowing to defend its South Korean ally with all its military capabilities, “including nuclear,” should war break out on the Korean Peninsula. About 28,500 American troops are stationed here.

Still, many in the country cannot shake the fear that they might one day be abandoned by the United States.

South Koreans questioned Washington’s commitment to the alliance when President Donald J. Trump demanded what they said were exorbitant sums for keeping American troops in the country. They watched in disbelief as the United States led a chaotic withdrawal from Afghanistan last year.

And as they witnessed Washington’s failure to prevent the Russian invasion of Ukraine, they wondered whether the United States would stop North Korea from invading the South, especially at the risk of leaving American cities and military bases in the Asia-Pacific more vulnerable to a nuclear attack.

“We don’t see global American leadership anymore. Instead, we rather find it feckless and helpless,” Lee Sang-min, a senior lawmaker affiliated with the governing Democratic Party, told a parliamentary hearing in February. “We even get skeptical whether we should rely entirely on the United States on issues that relate directly to our survival and prosperity.”

People in both Koreas view themselves as a small nation that has suffered numerous invasions and been occupied and divided by foreign forces. A once-common Korean saying advised: “Don’t trust the Americans and don’t be fooled by the Soviets, the Japanese will rise again and the Chinese will kill you — Koreans, be careful!”

Last week, Ukrainian officials warned that Russia may try to divide their country as Korea was divided after World War II.

Kim Jong-un, the leader of North Korea, has called his nuclear arsenal a “treasured sword” that will safeguard his country once and for all from foreign invaders. “We must be strong,” Mr. Kim said after resuming intercontinental ballistic missile tests in March. Only “nuclear war deterrence” will protect North Korea from “all threats and blackmails by the imperialists.”

Not long ago, similar ideas were popular in South Korea. In the 1990s, a novel titled “The Rose of Sharon Blooms Again” became a runaway best seller, with a plotline promoting nuclear nationalism.

In the book, the C.I.A. is suspected of assassinating a Korean nuclear physicist to stop him from building nuclear weapons, but South and North Korea join forces to build them — and deter another Japanese invasion of Korea.

“Who can guarantee that the Americans will remain our protector forever?” the protagonist, a newspaper reporter chasing the C.I.A. plot, says in the novel’s most famous line.

In real life, South Korea’s military dictator, Park Chung-hee, embarked on a covert nuclear weapons program in the 1970s, when the United States began reducing its military presence in the country. Washington forced Seoul to abandon the program, promising to keep the country under its so-called nuclear umbrella.

In 1991, the United States withdrew all of its nuclear weapons from South Korea, once numbering as many as 950, as part of the global nuclear arms reduction program. But Washington could not stop North Korea from building its own nuclear arsenal.

That has left South Korea facing three nuclear states to the north and west: North Korea, Russia and China.

“South Koreans wonder who would protect them if the United States bowed out,” said Lee Byong-chul, an expert in nuclear proliferation at Kyungnam University’s Institute for Far Eastern Studies in Seoul.

Calls for nuclear weapons have often bubbled up in South Korea over the decades, but they have never become a part of a mainstream political movement. President-elect Yoon Suk-yeol, who has promised to strengthen ties with the United States, has disavowed a nuclear-armed South Korea.

Washington fears that if Seoul were to build nuclear weapons, it would trigger a regional arms race and eliminate any hope of a nuclear-free Korean Peninsula. Secretary of the Army Christine Elizabeth Wormuth said last month that she “would be hesitant to contemplate” bringing nuclear weapons back to the peninsula.

Analysts like Mr. Cho, however, have argued it is time for Washington to boost South Korea’s confidence in extended deterrence. One possibility, they say, is to introduce a nuclear-sharing agreement with Seoul, similar to the one in which NATO aircraft would be allowed to carry American nuclear weapons in wartime.

When considering such options, South Koreans have more than a belligerent North Korea in mind: In the Carnegie Endowment survey, 56 percent of the respondents said that China would be “the biggest threat” to South Korea in the next 10 years.

If China were to invade Taiwan — the self-governing, democratic island that Beijing claims as its own — would North Korea, Beijing’s ally, see that as an opportunity to invade the South? And if Washington were facing conflicts in both Taiwan and South Korea, how would it respond?

Uncomfortable questions such as these have led to “greater calls for South Korea to actually have its own nuclear deterrent,” said Jenny Town, a senior fellow at the Stimson Center in Washington, during an online forum last month. “This is an issue that we’re really going to have to grapple with in the near future.”