We're Not Going Anywhere
From Halopedia, the Halo wiki
The track opens with muted percussion before high strings enter with an ambient and suspenseful melody, soon joined by low strings, then ends with cymbal clashes, echoing the opening of the Halo Theme.
The track plays in full during the Halo: Reach E3 2009 announcement trailer. The melody was adapted with additional choir accompaniment into Latchkey (from the earlier track ONI: Sword Base), which plays in the level ONI: Sword Base. A strings-only version was featured as City Under Siege (from the earlier track Exodus), which plays in the level Exodus.