The unit was involved in the delaying action against the Japanese invasion in the plains of Central Luzon, and the Battle of Bataan in 1942.

Mateo "Mattie" M. Capinpin (PA) as the division commander, Col. Nemesio Catalan as his Chief-of-Staff, and Col. Ray M. O'Day as Senior Instructor.

[2] The area of mobilization by the 21st Division covers the provinces of Tarlac, Pangasinan, La Union, and Nueva Ecija.

[3][1] Before Japanese units landed in Aparri and Vigan on December 10, 1941, the 21st Division under Gen. Jonathan Wainwright's Northern Luzon Force were instructed to dig in and hold the southern shores of Lingayen Gulf.

However, on December 22 at 0500 hrs he was surprised as the bulk of the invasion force consisting of the IJA 14th Area Army under Gen. Masaharu Homma landed in the eastern shores of Lingayen Gulf, avoiding the formation of the 21st Division, and focused on the fishing village of Agoo, La Union.

[5] The Japanese axis of advance would concentrate on Highway 3 which runs down from Agoo to Binalonan, Urdaneta, Tarlac, and Fort Stotsenburg.

[7] The 21st Division fell back to the Defensive Line D-1 on December 24, covering the area stretching from Aguilar to San Carlos in western Pangasinan.

It did not help that initial intelligence assessment outlined that both divisions were facing 120,000 Japanese units which had combat experience from the Second Sino-Japanese War.

The 23rd Infantry Regiment in reserve was able to blunt the probing attack, and by night time the general staff were planning for a counter-attack.

[6] The situation was also exacerbated by spies and local fifth columnist who exposed the locations of the 21st and 11th division units and supply dumps by firing flares into the night of 2 January.

[7] The following day, on 3 January, the 23rd Infantry Regiment moved into position to fill in the gap between the 21st and 22nd and was met with artillery fire and bombing and strafing from enemy air squadrons.

"[6] Gen. Capinpin launched a counterattack but the Japanese infantry supported by heavy artillery and dive bombers was able to push back.

[6] The 1st Battalion of the 21st Field Artillery covered the gap between the 21st and 22nd Infantry Regiment, firing almost at point blank of 600–800 meters at the advancing Japanese forces.

Major General Edward P. King Jr., the commander of Luzon Force in Bataan surrendered his battered, hungry, and sick troops on April 9, 1942 to prevent more death.