What is the history behind Imperium?

Imperium was published in 1992 - only 17 years ago, making this piece a 20th century composition.  So that must mean that this piece has no historical background, right?  Wrong!

Michael Sweeney borrows ideas from the music of the American Indians and uses them in Imperium
  • Percussion - To the American Indians, percussion is a very important part of the music.