De Facto The Matter
From Halopedia, the Halo wiki
De Facto The Matter is the nineteenth track from the Halo Wars: Original Soundtrack.
This track play during Halo Wars gameplay whenever the player commands their units to attack an enemy base, and was also used in the Halo Wars E3 2007 Trailer, which featured the early version of the game. It is played specifically at the last 2 minutes of the time-based campaign levels, Arcadia City and Escape.
This song's name is a play on words of the phrase "The fact of the matter".