On Jan. 5, one of the century’s most influential pop songs quietly celebrated its 10th anniversary. Yet surprisingly, if you turn back the clock to 2009 and take a look at the U.S. Billboard charts, that song is nowhere to be found, not even in the depths of the low 90s. The so-called Hallyu wave, the term for the rapidly expanding popularity of South Korean cultural artifacts around the world, had been impacting nearby China and Japan for a decade, but it had not yet crashed upon Western shores. But in 2009, when the wave did come to North America and Europe, the sound it made as it swept across millions of teenage hearts was a chirrupy, Auto-tuned “gee!”

The Korean girl group behind the “Gee” heard round the world, the catchily named Girls’ Generation, wasn’t aiming for crossover success. The group had only debuted two years earlier to a lukewarm reception in its homeland, and its first single sounded like the inoffensive Muzak that might accompany a daytime infomercial — hardly the stuff of a global, viral hit. It wasn’t exactly a humble beginning — the group’s management company, SM Entertainment, had already ruled the South Korean music industry for more than a decade, and Girls’ Generation was its new, crowning girl group. But the group was missing a runaway hit to achieve its reign atop the Asian music market.

But “Gee” wound up being so much more than that: Not only did it become an enormous hit in South Korea and Japan, but it also defined K-pop for curious Western listeners who stumbled upon the music video on YouTube, where it has more than 200 million views.

It’s easy to see the appeal of “Gee.” It begins in a dreamlike synth haze complete with glissandos on shimmering chimes, as if a fairy godmother were waving her wand. A voice rises from the treacle to introduce the conceit of the song (“my first love story”) with implications that stop just short of risque. Then a distorted “let’s go” bursts the reverie, and suddenly the listener is sent careening over a high-tempo bubblegum techno production. The girls sing sweetly about the anxieties of first love, but they do it while zealously riding the 4/4 beat.

This should sound familiar to anyone who lived through the ‘90s teen-pop explosion, best embodied by the early music of Britney Spears. The pop-music critic Ann Powers once said that “…Baby One More Time” made love sound like a sparring match. If that’s true, then Girls’ Generation makes love sound like a military parade, with the triumphant “gee”s of the chorus like each bullet of a 21-gun salute. And then, of course, there is the enticing music video, in which eight women coo and swoon in union, in accordance with the Korean term for childlike, quote-unquote “cute” behavior, “aegyo.”

And the comparison to Spears is apt: The techniques pioneered by Max Martin, the legendary Swedish songwriter behind Spears, Katy Perry and Ariana Grande, are all over the song. The pop-music theorist Owen Pallett compared Martin’s style to a “programming script” because his melodies are never “improvised on,” and “Gee”’s insistently staccato, repeating melodies have more than a whiff of the inhuman, the calling card of Martin’s perfectionist pop. Tellingly, “Gee”’s music video portrays the girls as store mannequins magically come to life.

The fact that “Gee” draws so heavily upon American teen pop music is not incidental to its Western success but perhaps the cause of it. Typically, when Korean pop acts such as BoA or Wonder Girls try to break into the U.S. music market, they attempt to create a more assertive, sexual tone and release songs with English lyrics. The result sounds like a Eurovision hack’s trend-chasing single, as if one plopped half an hour of top 40 radio into a NutriBullet — in other words, an utterly tasteless mess.

The nature of “Gee”’s appeal is different: It retains enough Western ingredients to be comprehensible and familiar to the American listener but rejiggers the recipe just enough to thrill. The sickly sweet synths, the indistinguishably polished voices, and the depressingly retro performance of femininity may sound strange to the Western ear, but there are kernels of familiar musical memories buried within. It all adds up to a Korean funhouse mirror in which the Westerner can see their own culture warped and reflected back at them, akin to reading a manga adaptation of the Batman comics or watching a Bollywood rip-off of a Hollywood caper.

Of course, “Gee” wasn’t a hit in the West, at least not by industry standards. But the song set a new precedent that later K-pop acts, such as Psy, BTS and Loona, would follow — their success would be measured in YouTube views, Twitter follows and Instagram likes, and not solely by chart peaks. After all, “Gee” was the most watched K-pop video on YouTube, at about 80 million views, until it was famously dethroned by Psy’s runaway novelty hit “Gangnam Style” in 2012.

But not even the members of Girls’ Generation themselves understood the elusive nature of their viral hit. When the group decided to capitalize on its unexpected success abroad and release a single, “The Boys,” targeting the Western market, the derivative, robotic result failed to crack the U.S. Billboard charts. To this day, its English-language version sits at a measly 200,000 views, while its Korean version has more than 200 million.

Still, a younger generation of K-pop artists would borrow from the “Gee” playbook to unprecedented success. Last year, when BTS’ album Love Yourself: Tear, notched, in a historic first for K-pop, the top spot on the Billboard 200, the group did it while singing in Korean. When Bang Si-hyuk, the head of BTS’ management company Big Hit Entertainment, was asked if the boys would ever sing in English, he said that fans “don’t require BTS to use English.”

Clearly, K-pop’s newest stars have realized that their appeal is rooted in their very divergence from the Western pop mainstream, and they have been working relentlessly to maintain that gap. I, myself, am not Korean or Korean American, but even from an outsider’s perspective, “Gee’s” success in the West through an embrace, rather than dismissal, of its Koreanness, seems to be an obvious factor in BTS’ rise to worldwide fame.

On the other hand, the aesthetic of “Gee” has become something that K-pop acts, specifically girl groups, have been struggling to escape. By introducing Western listeners to K-pop, it also created a one-dimensional image abroad of both Korean music and femininity — unrelentingly cheerful and simple for the former, innocent and cutesy for the latter. These aspects of Girls’ Generation’s image triggered a major backlash that marked a sea change in the history of K-pop. Today, K-pop’s girl groups have decidedly shifted away from performative innocence toward a more provocative attitude in both their music and appearance. But even as K-pop moves away from the conservative image of femininity that “Gee” so perfectly encapsulates, the song’s legacy as the first salvo of K-pop’s takeover of the global mainstream endures.