Finish the Fight is a song included in the Halo 3: Original Soundtrack. It plays during the Halo 3 Announcement Trailer and is 2:27 long.
It starts with a piano chord of eighths. Choir joins in with a soft melody, then piano plays another, louder chord. The choir then "replies" with another louder variation of their melody. The piano plays again, then the orchestra joins in, swelling louder and louder. Suddenly, it quiets again for a short piano motif. Strings then repeat the motif, reverting to a melody of strings, choir, and brass that is centered around the chord of E Minor. (Note: This section is more of a "multi-melody," where each instrument has their own melody. The result is an extremely complex binding of all the melodies together). At the end, brass and strings play the piano theme again, while the choir joins the harmony.
Martin O'Donnell, the composer of the song, discussed this piece with Chris Anderson of the Wired Magazine on April 23, 2007 here.