In 1942, the 123rd was reactivated as a component of the Illinois Army National Guard and fought in World War II where it received acclaim for its jungle fighting against the Japanese in the Pacific Theater.

Upon arrival in France, the 31st became a depot division, and all the recruits from its regiments were sent out to replace combat casualties in depleted units on the front.

Throughout the rest of 1942, and into March 1943, the regiment conducted thorough training at Fort Lewis until they moved to the Mojave Desert in early April 1943.

[2] They bivouacked near the Clipper Mountains and, along with other soldiers of the 33rd Division, had the distinction of being the first infantry trained in desert warfare without motorized vehicles.

The regiment was slated for a nine-month tour of duty as Kauai's defence force, taking over from elements of the 40th Infantry Division California Army National Guard, and began training.

When the Regiment left the area for combat on Luzon, the total number of enemy killed in that sector, including those accounted for by preceding troops, had been swelled to 4,005.

[2] The most bitter battle of the Sarmi area took place when Company C of the 1st Battalion (1-123), led by CPT Marchant, encountered heavy resistance at Sawaar Airfield.

In the ensuing skirmish, which lasted all day, all three enemy tanks were knocked out, one of them by a well-aimed bazooka shot fired by Captain Marchant,[2] and 25 Japanese dead were found laying around the jungle.

Lieutenant Walter Roper, leading a patrol deep in enemy territory, was the first man in both the regiment and the division to lose his life in combat.

[2] The Regiment left Maffin Bay on 26 January 1945, joined other elements of the 33rd Division en route, and departed for Luzon in the Philippines.

[2] Unlike in Europe, it was common for aid-men to carry weapons in the Pacific Theater because the Japanese did not follow the Geneva Conventions and often killed Allied medics with indiscretion.

The Japanese were making a valiant defense of this area as it controlled the last water points on the routes leading to Baguio.

During the fighting in the mountains, units of 2-123 executed a brilliant night operation to capture strategic points on the route of advance.

This night attack was so well undertaken that the Japanese were unaware that their strong emplacements had been occupied, and at daybreak the enemy was mowed down in great numbers as they emerged from their caves on the reverse side of the hill to man their positions.

[2] In the fight to the highest point of terrain, K Company 3-123 launched a well-coordinated attack after first having an artillery barrage laid down less than a hundred yards to the front.

Timed to perfection, the assault, which immediately followed the lifting of the barrage, caught the Japanese in their foxholes and the victory was achieved without any casualties.

3-123 nicknamed the brushy area near Baguio "Times Square" due to the heavy activity of enemy vehicles, including tanks.

Assembling near Vigan, Connolly Task Force would follow Route 3 around the northwest tip of Luzon to the vicinity of Aparri, where a battalion of the 11th Infantry would join.

[4] On 11 May 1945, a member of this force, PFC John R. McKinney, singlehandedly killed 40 Japanese soldiers in a half hour battle despite being wounded by a katana strike to the head.

Gen. Krueger's estimate of the Japanese situation dictated the need to make a vertical envelopment of airborne troops to close the trap and prevent the enemy from all possibility of escaping from Aparri.

[5] Despite the great successes of the now reinforced Connolly Task Force, Krueger did not change his mind about the desirability and necessity for the airdrop.

Instead, he concluded that the "seizure of Aparri without opposition” by elements of the Connolly Task Force on 21 June 1945, together with the almost unopposed advance of the 37th Division, indicated clearly that the time had come for mounting the airborne troops to “block the enemy's retreat in the Cagayan Valley."

[6] The 123rd Infantry Regiment as a whole took part in some of the longest and bloodiest campaigns of the US Army during World War II and it suffered a total of 96 combat casualties.