The intro to Metallica's “Don't Tread on Me” interpolates the melody of “America” from the musical West Side Story. Leonard Bernstein composed that melody for the stage work.
The song appears on The Black Album as a compact, anthem-like track built around a patriotic reference to the Gadsden flag and its “Don’t Tread on Me” motto. Its opening musical quotation is easy to miss because it is integrated into a heavy-metal arrangement rather than presented as a separate orchestral passage.
West Side Story premiered on Broadway in 1957, with music by Bernstein, lyrics by Stephen Sondheim, and a book by Arthur Laurents. The musical’s “America” is one of its best-known numbers, making the source melody recognizable to listeners familiar with the show.
This fact is sometimes described as a sample, but “interpolates” is more precise for the album’s credited musical borrowing. The quotation is part of the song’s introduction, not a separate cover version of “America.”