From Halopedia, the Halo wiki
The 124th Infantry Brigade (map designate: MCH 124 X) was a mechanized infantry formation of the UNSC Army during the Human-Covenant War. This brigade was garrisoned in the Viery territory of the Human colony of Reach. It participated during the invasion to counter Covenant Army landing forces.
At the time of the assault on Viery by the Covenant invasion forces, the 124th Mechanized had six regiments.