The 21st Infantry (Invincible) Battalion became the first organic tactical maneuver battalion of the then 5th Infantry Brigade (Sep), Phil Army, now the 5th Infantry Division. It was reactivated from the defunct 21st Battalion Combat Team (BCT).
This unit was first deployed in Mindanao and has been seen in action in Cotabato, Zamboanga and Basilan. Immediately after its activation and training at Fort Magsaysay, Palayan City. The battalion was recalled and dispatched to continue combat assignment in Luzon after three (3) years of combat duty in Mindanao. From 1979 to date, the battalion has covered almost every barangay of Region 2, the Central Plain of Luzon and the Cordillera Administrative Region (CAR).