Mary J. DiMeglio

DOVER, Delaware -- Imagine Dragons unleashed their 'Battle Cry' on the U.S., using their Saturday night Firefly Festival slot for the tune's live U.S. debut.

"We're gonna do a song we've never done in the country," singer Dan Reynolds announced. "We played it three days ago in Hong Kong. We were in the future, because Hong Kong is in the future."

They'd performed the "Transformers: Age of Extinction" theme song at the film's worldwide premiere in Hong Kong, one of the locations in the film.

Backed by pummelling drums at Firefly, Reynolds turned up the drama, wailing, "Nobody can save you now/ It's do or die."

The Las Vegas band was oozing with gratitude for its fans, who have helped them evolve from opening act to prime time.

"It's such a beautiful evening," Reynolds shared. "Last time we were here -- we played Firefly two years ago -- it was at 11 a.m., and to be back here now doing this again, we can't tell you how much it means to us. Thank you for letting us do this. You are the greatest people in the world."

Imagine Dragons are still performing strong in support of their acclaimed debut, 2012's Night Visions. "Tiptoe" and "Rocks" made the set, and "On Top of the World" was a huge highlight, as was "Who We Are," from the "Hunger Games: Catching Fire" soundtrack.

The band gave love to Saturday's other acts, saying they were honored to be playing alongside "legendary artists" Outkast and Beck. They also paid tribute to one of their favorite bands, busting out Rush's "Tom Sawyer."

"I believe music is the greatest communicator there is," Reynolds said, launching into the fun bop of their first single, "It's Time."

Going out with a bang, Cage the Elephant joined the guys onstage to sludge through their ubiquitous radio hit "Radioactive."