- This song addresses the topic of patriotism. Front-man and chief songwriter Billie Joe Armstrong: "It brings up 21st Century Breakdown in a lot of ways, and the 21 gun-salute for someone that's fallen, but done in an arena rock 'n' roll sort of way."
- At the pre-release preview of 21st Century Breakdown, the MTV News reviewer called this song, "a cell-phones-in-the-air anthem."
- Marc Webb (Maroon 5, My Chemical Romance) directed the music video, which features a fugitive bank-robbing couple holed up in a room with Green Day lyrics scrawled on the walls. To stage the shoot-out, a thousand tiny explosives called squibs were embedded in the walls and furniture to mimic gunfire, and the crew shot marbles from guns to shatter the fishtank. No fish were harmed - they were swapped out for rubber ones - but the actors (Josh Boswell and Lisa Stelly) were left battered and bruised. He was hit by shrapnel from the exploding room while she received a squib blast that drew blood.
- It was nominated for a Grammy for Best Rock Performance by a Duo or Group with Vocal and Best Rock Song in 2010.
- The song is written in the key of F Major/D minor. John Phillips has a writing credit due to the similarity of the verse melody to the 1967 song San Francisco (Be Sure to Wear Flowers in Your Hair).
- The song debuted at No. 55 on the Billboard Hot 100, peaking at No. 22 (becoming their highest peaking song since the 2005 hit, "Wake Me Up When September Ends."
- The song is featured on the soundtrack of the film Transformers: Revenge of the Fallen and is featured as one of the songs during the end credits.
Green Day has recorded two versions of "21 Guns" with the cast of the musical American Idiot (not to be confused with the 2004 Green Day album of the same name). Armstrong, who also produced the recording, is singing the male solo parts. The second version of the song with the cast of the musical American Idiot features Stark Sands, John Gallagher Jr., and Michael Esper singing their respective solos instead of Armstrong.
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