Florida Georgia Line's ACM Awards performance with the Backstreet Boys was the most tweeted-about moment of Sunday night, but it was a long time coming. Here are six things to know about the groups' past and future together.

Pop moments like this always upset country traditionalists, but they're inevitably what everyone is talking about after the show. Need proof that country vets approved of FGL and BSB? Check out Tim McGraw's reaction:

The two groups are longtime admirers of one another, but the genesis of this night started at CMA Festival a year ago. What began as a secret recording process became a lifelong friendship, and perhaps even more — Florida Georgia Line have said that Nick Carter and company are becoming mentors.

At the 2017 ACMs, the group started with "God, Your Mama and Me" from FGL's Dig Your Roots album and then opened up "Everybody (Backstreet's Back)" ... and the T-Mobile Arena erupted. Stars young and old stood, just like they did for Beyonce at the 2016 CMAs and Justin Timberlake in 2015. When Tyler Hubbard and Brian Kelley started doing the Backstreet's Back dance ... well, you saw McGraw's reaction. He — and everyone — was floored.

Look for the mega-collaboration to continue. Both acts are confirmed for 2017 CMA Music Festival, and they'll play stadium dates on the Smooth Tour this summer. Nelly and Chris Lane are also part of those shows.

See the Best Pics from the 2017 ACM Awards Red Carpet