When called into service for World War II, the 103rd became part of the 43rd Infantry Division, the "Winged Victory" Division.
 After finishing training at Camp Shelby, Mississippi, they were sent to the Pacific Theater to fight against the Empire of Japan.
 They embarked for New Zealand on 1 October 1942, and arrived at Auckland three weeks later.
 The first combat operations undertaken by the regiment were in early 1943 on Guadalcanal.
 The Japanese had ended all organized resistance but mopping up operations were conducted to root out stubborn stragglers.
 The first major action the 103rd Infantry took part in began on 22 June 1943 in the New Georgia Campaign.
 The New England infantrymen did battle with the Japanese and the jungle alike, and many men fell to disease.
 Here, the soldiers "were soon introduced to the harsh realities of jungle warfare".
The battle was tough for the new regiment and they became bogged down in the dense jungle by the savage fighting against the enemy.
 The 43rd Division made a westward advance against the Japanese airfield at Munda and covered the southern (coastal) flank of the drive.
 The Battle of Munda Point was extremely bloody for the regiment; progress was slow and casualties were high, but eventually the area was captured on 5 August 1943, effectively ending all organized Japanese resistance on the island.
 The 103rd saw extensive and bloody combat at New Georgia and was sent to New Guinea to act as a reserve unit during the New Guinea Campaign, where it saw minor action as a supporting element.

 In the Philippines, the 103rd Infantry Regiment landed on the island of Luzon on 9 January 1945 and was one of the leading elements during the Battle of Luzon.
 As the left wing of the invasion force, they pushed inland, encountering stiff opposition from fanatical Japanese enemies.
 The 103rd fought for about a month in the breakout operations from Lingayen, and finally received a week long rest in February.
 They were sent back to the front to mount a surprise attack against the Japanese Shimbu Line.
 They then moved into the highlands region of the island and engaged the enemy in fighting that was often cave-to-cave.
 On 9 May 1945, they secured the Ipo Dam from the enemy, which controlled roughly 30% of Manila's water.
 Both the 103rd Infantry and the 152nd Field Artillery Regiment (both from the Maine Army National Guard) received the Philippine Republic Presidential Unit Citation for their daring actions during the Battle of Luzon.
 In 1959, the regiment converted from infantry to armored cavalry, becoming the 103d Armored Cavalry Regiment.
In 1961, it again changed to the 20th Armor Regiment.
In 1967, the 20th Armor converted to the 133rd Engineer Battalion, which now carries the lineage of the 103rd Infantry Regiment.
When called into service for World War II, the 103rd became part of the 43rd Infantry Division, the "Winged Victory" Division.  After finishing training at Camp Shelby, Mississippi, they were sent to the Pacific Theater to fight against the Empire of Japan.  They embarked for New Zealand on 1 October 1942, and arrived at Auckland three weeks later.  The first combat operations undertaken by the regiment were in early 1943 on Guadalcanal.  The Japanese had ended all organized resistance but mopping up operations were conducted to root out stubborn stragglers.  The first major action the 103rd Infantry took part in began on 22 June 1943 in the New Georgia Campaign.  The New England infantrymen did battle with the Japanese and the jungle alike, and many men fell to disease.  Here, the soldiers "were soon introduced to the harsh realities of jungle warfare". The battle was tough for the new regiment and they became bogged down in the dense jungle by the savage fighting against the enemy.  The 43rd Division made a westward advance against the Japanese airfield at Munda and covered the southern (coastal) flank of the drive.  The Battle of Munda Point was extremely bloody for the regiment; progress was slow and casualties were high, but eventually the area was captured on 5 August 1943, effectively ending all organized Japanese resistance on the island.  The 103rd saw extensive and bloody combat at New Georgia and was sent to New Guinea to act as a reserve unit during the New Guinea Campaign, where it saw minor action as a supporting element. 
 In the Philippines, the 103rd Infantry Regiment landed on the island of Luzon on 9 January 1945 and was one of the leading elements during the Battle of Luzon.  As the left wing of the invasion force, they pushed inland, encountering stiff opposition from fanatical Japanese enemies.  The 103rd fought for about a month in the breakout operations from Lingayen, and finally received a week long rest in February.  They were sent back to the front to mount a surprise attack against the Japanese Shimbu Line.  They then moved into the highlands region of the island and engaged the enemy in fighting that was often cave-to-cave.  On 9 May 1945, they secured the Ipo Dam from the enemy, which controlled roughly 30% of Manila's water.  Both the 103rd Infantry and the 152nd Field Artillery Regiment (both from the Maine Army National Guard) received the Philippine Republic Presidential Unit Citation for their daring actions during the Battle of Luzon.  In 1959, the regiment converted from infantry to armored cavalry, becoming the 103d Armored Cavalry Regiment. In 1961, it again changed to the 20th Armor Regiment. In 1967, the 20th Armor converted to the 133rd Engineer Battalion, which now carries the lineage of the 103rd Infantry Regiment.
103rd Infantry Regiment (United States)