Wonder Girls are true pioneers of K-Pop.
You see, in 2009, this song entered the Billboard Hot 100 at number 76. Modern K-Pop fans take for granted that there will be South Korean artists on the pop charts. Here's the list of all the artists that were on the Billboard Hot 100 from South Korea before Wonder Girls:
Oh. Wait. There weren't any.
NOT ONLY was this the FIRST South Korean song to be a US pop hit, it would go on to become the best selling physical single of 2009 (in an era when the physical single was dying a quick death). The song itself is a bit of a Motown throwback in its harmonies.
The version above is the one that hit the US charts. Of course they did a Korean version, and of course it was a hit in their home country.