GYERYONG, South Korea >> The audience applauded politely as the honor guard twirled its rifles, and oohed and ahhed at the acrobatic taekwondo demonstration. But the real excitement began when the army band appeared.

The big draw: Jung Yun-ho, a soldier otherwise known as U-Know Yunho, one of South Korea’s most famous K-pop stars. He burst onto the stage of a military runway here in the mountains south of Seoul, wearing a black T-shirt emblazoned with “Korea Army” on the back.

“This is the band I organized in the army,” Jung shouted, gesturing to the other soldier-musicians behind him as 2,500 civilian fans quivered and shrieked in delight.

The audience waved red balloons and paper fans with pictures of Jung’s face as the group launched into “Mirotic,” one of many hits by TVXQ, the pop act in which U-Know performed before he began serving his mandatory term in the army.

While armies around the world have marching bands and musical troupes, South Korea may have the trendiest ones of all.

In South Korea, where every able-bodied man 18 to 35 years old must complete a 21-month stint in the armed forces, the rules make no exceptions for pop idols, no matter how much their fans may miss them and how much income they stand to lose while enlisted. And in a nation where powerful talent agencies routinely recruit new singers and dancers to create pop groups, there is a constant stream of celebrities eligible for military duty.

About 630,000 soldiers are serving active duty, a vestige of the Korean War and a continuing reminder of the country’s vigilance against the hostile nation to the north.

The corps includes a handful of K-pop celebrities whose prime performing years fall within the conscription window. The stars used to be routinely assigned to a separate celebrity unit, but the army disbanded it three years ago, after a string of public scandals involving K-pop soldiers.

Today, most of the stars serve in military bands that perform periodically for their fellow soldiers. But once a year, the army hosts a six-day propaganda bonanza where civilian fans can see their favorite pop idols free of charge. The fans are also treated to hand-to-hand combat drills, parachute landings and displays of tanks, rocket launchers and Chinook helicopters.

Many citizens resent military service as an unwanted interruption to the education and careers of the country’s young men. With some politicians also debating whether the military should convert to a voluntary service, the K-pop bands serve as syrupy agitprop and a potential recruiting tool.

“The military can come across as a scary organization,” said Col. Lee Jong-eung, director of the annual festival, which drew more than 1 million visitors this month. “But when these celebrities come to the army, everybody knows them, and we are asking them to unleash their talent to soften the image of the army.”

K-pop has also been used in the campaign of psychological warfare that South Korea has waged against North Korea. Last year, South Korea blasted pop songs by acts like Apink and BigBang over loudspeakers in the Demilitarized Zone, leading the North to threaten “all-out war” if the broadcasts did not cease.

At the same time, K-pop music has been one of South Korea’s most successful exports, helping to link fans across Asia and beyond. Fans traveled from Japan, China, Germany, Hungary and Morocco to see the K-pop performers at the military festival this year.

“It’s easy sociocultural currency for Koreans to transmit outside of their borders,” said Katharine H.S. Moon, a professor of political science at Wellesley College. “In the north, it’s used as a weapon to stick it to the enemy, and within its own nation and among friends and fans, it’s a very positive bonding tool.”

Yet K-pop stars have had a checkered record in the army, with some trying to evade conscription and others taking advantage of their celebrity status to flout army rules. Although military service may be unpopular in South Korea, the public expects all men to fulfill their civic duty and harshly judges those who do not. (Women are not required to serve.)

In 2002, Yoo Seung-jun, one of South Korea’s biggest-selling pop artists, obtained U.S. citizenship just a few months before he was scheduled to enlist in the army. He was barred from South Korea and just last month lost a lawsuit in which he demanded the right to re-enter the country.

Two K-pop performers were jailed for 10 days two years ago after they were arrested during their military service for visiting massage parlors that also sold sexual services. Another star soldier was disciplined after he sneaked out to visit his girlfriend, a famous actress, while on duty.

Many in the public suspect the stars are still afforded privileges denied the typical soldier. At the festival this month, the five celebrity performers retreated to an air-conditioned tent for bottles of green plum juice after the show, while the backup band members sat backstage on the asphalt eating snacks.

An army spokesman denied repeated requests to interview any of the celebrities, saying their images were still protected by the talent agencies that represent them as civilians. A spokeswoman for SM Entertainment — the agency that represents U-Know Yunho and three members of the K-pop band SuperJunior who are in the army and performed at the festival — said that because of their army service the pop artists would not be available for interviews.

Cho Gae-hyuk, a soldier who composed a short musical about the Korean War for the festival and shared a barracks room with five of the K-pop stars for four months, said he was surprised by their work ethic.

“We were with them every day,” he said. “They are very disciplined and are more like soldiers than the younger soldiers.”

Observers of the K-pop scene say some of the stars shrewdly use army service to their advantage. Although South Koreans may not like the military draft in reality, they romanticize the army culturally, gobbling up pictures of their idols in uniform. “Descendants of the Sun,” a TV melodrama about a special forces captain and his doctor girlfriend, was a huge hit when it aired this year.

Lee Seung-gi, a solo pop artist who served as master of ceremonies at the festival, went further than most of his peers and is serving in a special forces unit. He told fans that he had overcome his fear of heights during a parachute jump in army training.

“Outside the military, I would not have challenged myself to do these things,” Lee said.

Some fans who had come primarily to see their idols — without the typical $100-plus cost for concert tickets — came away impressed by the more overtly military demonstrations.

“Normally we don’t know what the soldiers do, and we always thought the image of the soldiers were so dry and scary,” said Park Eun-kyung, 36, a fan of U-know Yunho. “But they seem to be so hard-driving. I felt like my impression of them had softened.”

© 2016 The New York Times Company