BTS go off into the army - what now for K-pop's biggest stars?

Imagine if the Beatles broke up at the height of their fame to join the army.

That's what the members of BTS, the world's biggest pop band, are doing right now.

On Tuesday, lead vocalist Jung Kook joined the rest of his K-pop band members and enlisted for military service - a requirement of all able-bodied South Korean men aged 18-28.

Just four weeks ago, he was riding the high of his crossover solo career in New York.

He blew the socks off Tonight Show host Jimmy Fallon with a scintillating Michael Jackson-esque song and dance. The following night he performed a last-minute public concert that went viral across TikTok. to screaming crowds from a Times Square rooftop.

In the past few weeks, he's released collaborations with Justin Timberlake and Usher, his debut album Golden hit number one on several charts, and the dance solo for his hit single Standing Next to You has become a TikTok trend.

But just as he was soaring into the next stratosphere of stardom, he hit the brakes and returned to Seoul.

A few days later, the 26-year-old and three other BTS members held a pizza party livestream, where they told fans the time had come to follow three other band members into military duty.

As South Korea is still technically at war with its hostile neighbour North Korea, most men are required to do an 18 month stint in the army.

But there had long been a debate over whether BTS, arguably South Korea's most famous cultural export, would have to serve as well.

There had been exemptions given previously to Olympic medallists and classical musicians, and in 2020, the South Korean parliament passed a bill allowing BTS to delay their compulsory military service until the age of 30.

Defenders of the band, including then government ministers, argued that Korea's biggest pop stars had already served their country by earning it billions of dollars and they should be allowed to continue in their superstar capacity.

But then last October, their agency BigHit Music owned by HYBE, confirmed that all seven members would fulfil the obligation, starting with the eldest, Jin, who would join in December 2022.

The band would go on a hiatus - to accommodate military service and to also allow members time to pursue their own projects.

"For Western audiences, it does seem quite cruel that people at the height of their success have to stop and take a forced hiatus whether they like it or not," says K-pop academic Grace Kao, a professor at Yale University.

But it's a reality that many in South Korea are used to, she says. BTS follows in the footsteps of the scores of other K-pop idols and K-drama stars who've had to take time out for the military.

And with the advanced warning, their fans globally had been steeling themselves for this moment. "It was not a surprise," Prof Kao says.

Still that hasn't made the actual day of departure any less bittersweet. BTS' reach is reflected in the range of languages - from Spanish to Vietnamese - in which fans wrote emotional tributes this week.

As the four remaining members - RM, V, Jimin and Jungkook - were shown heading off to camp on Monday and Tuesday, the comment threads proliferated with crying emojis.

"All of them gone at once," one fan wrote on TikTok. "My heart is hurting OMG, I just can't do this anymore. I'm gonna miss them all so much."

BTS' fandom, known as ARMY - an acronym for Adorable Representative MC for Youth- have been measured to be the most engaged social media fanbase of any artist in recent years. Malaysia-based K-pop academic Jimmyn Parc told the BBC he believes many may be experiencing a short "depression".

On Monday and Tuesday, a lesser known single from six years ago, Spring Day, suddenly surged to the top of the US' iTunes charts.

"ARMY is charting BTS songs in BTS's absence...I've always appreciated this show of love from the fandom," one fan wrote on the band's main Reddit sub-forum, which has over 610,000 members.

The tone struck has been reminiscent of WW2 wives sending their sweethearts off to the war. Many BTS fans say they have already pledged total loyalty.

It's this level of sheer dedication - from the biggest fanbase in the world - that will most likely sustain the band's status, industry watchers say.

"Generally speaking, no matter where you are in the world, if a musical group has a hiatus, it affects their popularity. But I'd venture to say if any group could buck that trend, it would be BTS," says Jeff Benjamin, a K-pop columnist and writer at Billboard Magazine.

Key to this is management's strategy of maintaining a steady feed of content.

"There have been songs, videos, photoshoots, fan messages and much more all prepared by the members before their enlistment. These kinds of things are super important to continue that support when most groups will need to go quiet at the time," says Mr Benjamin.

Meanwhile for the rest of the thriving K-pop industry, BTS' absence offers opportunities for the other bands breaking through. Several have already made it onto Western mainstream charts - listeners having cottoned on to bands like New Jeans, Le SSerefim.

"BTS was too focused on by media outlets," says Associate Prof Parc. "This pause will give other K-pop groups changes to receive more of the limelight. It's a win-win situation for the industry."