Recording and composition
"One" was written in November 1987 by Metallica's principal composers James Hetfield and Lars Ulrich. The song was released in 1989 as the third and final single from the album. For the first 17 seconds of the song there are a series of sound effects with a battle theme, an artillery barrage and helicopter are heard and continues slightly over a clean tone guitar intro by Hetfield before Kirk Hammett comes in over the top with a clean-toned solo. Ulrich's drums come in and continues until each chorus, when the guitars become heavy and distorted before returning to clean. There is a second solo by Hammett halfway through the song, before lyrics cut out and the song gradually gets more heavy and distorted until the "machine gun" guitar build up (played alongside double bass drums) before the next, often highly praised, guitar solo by Hammett, and a final dual solo by Hammett and Hetfield. The song begins in 4/4 time, and later 3/4 as well as 2/4.
In 1991, James Hetfield told Guitar World that he wrote the song's opening B-G chord change (miscalling it a 'modulation') based on an idea prompted by the Venom song "Buried Alive" from their second studio album, Black Metal.
I had been fiddling around with that B-G modulation for a long time. The idea for the opening came from a Venom song called "Buried Alive". The kick drum machine-gun part near the end wasn't written with the war lyrics in mind, it just came out that way. We started that album with Mike Clink as producer. He didn't work out so well, so we got Flemming to come over and save our asses.
The song starts off in a soft melodic setting, but it develops through multiple sections into heavier and faster speed metal sounds, leading up to a technically complex tapping solo by Kirk Hammett, and a dual guitar section by Hammett and James Hetfield.