From Halopedia, the Halo wiki
Fall of Reach
1 Alpha was a UNSC Army unit stationed on Reach during 2552. During the Fall of Reach on August 23, they fought in the Siege of New Alexandria, where 1 Alpha 3 and her team became pinned down by Covenant fire on a rooftop in New Alexandria. However, SPARTAN-B312 was able to relieve them.