Which musical work is interpolated in the introduction to Metallica's “Don't Tread on Me”?

The introduction to Metallica's “Don't Tread on Me” interpolates “America” from West Side Story.

“America” was composed by Leonard Bernstein for the 1957 musical West Side Story. Metallica uses the recognizable melodic idea at the start of “Don't Tread on Me,” creating a musical connection between a classic Broadway work and a heavy-metal album released in 1991.

The song's title and imagery also connect to the Gadsden flag, whose snake and “Don't Tread on Me” motto appear in the album's packaging. That relationship is why the track's introduction is often discussed alongside the Black Album cover rather than as an unrelated musical quotation.

Interpolation means that a pre-existing musical passage is incorporated into a new composition; it is not the same thing as simply naming a song or using a recording of the original performance. The relevant source is Bernstein's “America” from West Side Story, not the national anthem or another musical-theater number.