The 21st Infantry Battalion dates back from 1 March 1973 when the “Invincible” Battalion became the first organic tactical maneuver battalion of the then 5th Infantry Brigade (Separate), Philippine Army. The units lineage traces back to the 21st Battalion Combat Team, which was reactivated. When the 5th Infantry Brigade was expanded into a full division, the 21st Infantry Battalion became part of the 1st Infantry Brigade, 5th Infantry Division (subsequently the 501st Infantry Brigade, 1st Infantry Division).

The unit was first deployed in Mindanao and saw 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 3 years of combat duty in Mindanao. From 1979 through 2009, the Battalion had covered almost every barangay of Region 2, the Central Plain of Luzon and the Cordillera Administrative Region.
In the last quarter of 2014, the unit once again set its foot to its origin in the land of Promise where it continue to flourish the Army mission within Zamboanga Peninsula

 dates back from 1 March 1973 when the “Invincible” Battalion became the first organic tactical maneuver battalion of the then 5th Infantry Brigade (Separate), Philippine Army. The units lineage traces back to the 21st Battalion Combat Team, which was reactivated. When the 5th Infantry Brigade was expanded into a full division, the 21st Infantry Battalion became part of the 1st Infantry Brigade, 5th Infantry Division (subsequently the 501st Infantry Brigade, 1st Infantry Division).

The unit was first deployed in Mindanao and saw 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 3 years of combat duty in Mindanao. From 1979 through 2009, the Battalion had covered almost every barangay of Region 2, the Central Plain of Luzon and the Cordillera Administrative Region.
In the last quarter of 2014, the unit once again set its foot to its origin in the land of Promise where it continue to flourish the Army mission within Zamboanga Peninsula