A survey last year by the government-run Korea Institute for National Unification in Seoul found that far more South Koreans in their 20s now oppose reunification — 71.2 percent — than support it. Across the population, support has dropped to 57.8 percent from 69.3 percent just four years ago.
“Especially men in their 20s, about half of them, consider North Korea an outright enemy,” said Kim Ji-yoon, a research fellow at the Asan Institute for Policy Studies in Seoul who has been tracking attitudes toward reunification. “To young South Koreans, North Korea is someone they don’t want anything to do with.”
Polling experts say that South Korean men in their 20s often get more hawkish after they finish their mandatory military service.
The skepticism was apparent this month, when the two Koreas set aside their disputes over the North’s nuclear weapons and missile programs and agreed to field a joint team in the Winter Games in the town of Pyeongchang, and to march together in the opening ceremony on Feb. 9.
In the past, such gestures triggered waves of pro-unification sentiment, as in 2000, when North and South Korean athletes marched together at the Games in Sydney, Australia. (They competed separately.) The administration of President Moon Jae-in, a progressive who has long supported inter-Korean unity, hoped this latest rapprochement would create similarly warm feelings.
Instead, a survey found that more than 72 percent of South Korean adults overall — and more than 82 percent of those in their 20s and 30s — were not enthusiastic about the hockey team. More than 54,000 people signed a petition opposing it, and many expressed anger that some South Korean players would cede their positions to North Koreans.
“I am taken aback,” said Kim Sung-hwan, a former South Korean foreign minister. “Young people seem to think of North Korea as strangers who barge into their party bringing with them nothing but empty spoons.”
Such pushback would have been expected from conservatives, who have long been suspicious of efforts to engage the North. But younger South Koreans tend to be politically progressive and supportive of Mr. Moon on other issues.
Analysts said that years of increasingly provocative nuclear and missile tests have darkened South Korean perceptions of the North and its young leader, Mr. Kim. Actions like Mr. Kim’s executions of his own uncle and his half brother have also made the regime look brutal and grotesque.
“We know that this sudden shift in mood could be confusing for people, especially given how unsettled they were over the North’s missile tests up until just a month ago,” said Yoon Young-chan, a spokesman for Mr. Moon. “But the Pyeongchang Olympics can go beyond South-North reconciliation and provide clues to easing tensions and building peace on the Korean Peninsula.”
Analysts say a key turning point in attitudes toward the North came in 2010, when a South Korean naval ship was sunk by an apparent North Korean torpedo attack, killing 46 sailors, and North Korea launched a rocket barrage on a South Korean island that killed four people, including two civilians. These were formative events for young South Koreans, these analysts say.
Key members of Mr. Moon’s presidential office and governing party are progressives in their 50s, who went to college in the 1980s. Then, campuses were rife with anti-American activism, partly driven by resentment over the division of the Korean Peninsula after World War II. Students defied the authorities by sending a “unification envoy” to the World Festival of Youth and Students, alternative games that North Korea held in Pyongyang in 1989 to counter the previous year’s Summer Olympics in Seoul.
Progressives in that era believed in a peaceful process of reunification, built on the expansion of economic and social exchanges. Today, many of that generation see the North’s nuclear weapons program as a desperate attempt to protect itself from the United States and the South, with which it is still technically at war.
Reunification is also a personal matter for Mr. Moon, 65, who was born in a refugee camp after his parents fled their native North Korea during the 1950-53 Korean War.
“If Korea reunified, the first thing I would do is to take my mother’s hand and visit her hometown,” he said during last year’s election campaign.
The assumption that the South and North belong together is shared by many South Korean conservatives, though from a very different point of view. They tend to call for the South liberating the North from the Kim family’s rule, by force if necessary.
The horrors of the Korean War, which killed millions, bound older South Koreans together. Many still grow teary-eyed on those occasions when the rival governments allow select groups of aging citizens to meet relatives from the other side after decades of separation.
Younger South Koreans don’t share the pain caused by the peninsula’s divide, or the inclination to see North Koreans as long-lost brethren. Last year’s survey by the unification institute found that while more than 47 percent of respondents in their 60s and older said the two Koreas must reunify “because they belong to the same nation,” less than 21 percent of respondents in their 20s said so.
Of course, warier views of the North are not limited to the young. Many South Korean conservatives, young and old, embrace the view that the North is building nuclear-tipped intercontinental missiles to drive a wedge between it and the United States, its longtime protector.
They fear that if Mr. Kim thinks the threat of a nuclear strike on the mainland United States will keep the Americans from intervening, he may try to take over the South, the dream that his grandfather, Kim Il-sung, the North’s founding leader, failed to achieve during the Korean War.
On Thursday, North Korean state media called on “all Koreans at home and abroad” to support moves toward reunification and eliminate obstacles created by “outside forces,” a reference to the United States.
Many South Koreans are relieved that North Korea is joining the Olympics, because it is less likely to conduct a major weapons test while its own athletes are competing in Pyeongchang. But they remain deeply skeptical about Mr. Kim’s sudden peace offensive.
In a town near the Olympic venues, Kang Hee-du, 54, a restaurant worker, wondered how the Koreas could possibly reunify when they have drifted so far apart, economically and ideologically. “It would be nothing but chaotic,” he said.
But Choi Sang-hwan, 73, a retired auto-component maker, lamented the new attitude.
“Our generation knows how tragic war can be; it can take everything away,” said Mr. Choi, who is also an Olympic volunteer. “It’s our duty to let our future generations live peacefully in a unified Korea.”