The battalion was created in 1918 in the District of Columbia National Guard. It was mobilized into Federal service in 1940 as part of the 29th Infantry Division, made up of units from Maryland, Virginia, and the District of Columbia. The 121st staged at Fort Meade, Maryland for movement to England. At the same time, the 37th Infantry Division from the Ohio National Guard, was staging at Fort Indiantown Gap, Pennsylvania.
The 37th has also been alerted for movement to England, and had sent its 112th Engineer Combat Battalion ahead as part of the advance party. Orders were changed and the 37th was diverted for service in the Pacific Theater. There was no time to recall the 112th, or to create and train a new engineer battalion. The War Department ordered all personnel and equipment of the 121st Engineers moved from Fort Meade to Fort Indiantown Gap, and the unit was redesignated the 117th Engineer Combat Battalion. One officer and six enlisted personnel, symbolically representing the 121st Headquarters, each line Company, and the Medical Detachment, remained behind with the organization's colors.
The new 117th Engineers shipped out to the Fiji Islands, and saw extensive combat in the Philippines. The men from the DC Guard worked under enemy fire building and repairing 64 bridges, destroying enemy held buildings and tank obstacles, and participating in river crossings with "consummate skill and courage."
When the 29th Division reached England, the 112th Engineer Battalion from the 37th Division became the reconstituted 121st Engineers. By the time the 121st saw its first combat, on D-Day at Omaha Beach during the invasion of Normandy, its ranks consisted of the soldiers from Ohio as well as new soldiers from throughout the United States. None of the seven original DC Guardsmen were with the unit at the time of the invasion.
The battalion remained active until May 1945 in operations throughout Europe.
The history, lineage and honors of the 121st continue today in the lineage of the 372d MP Battalion of the District of Columbia Army National Guard. In 1948, a "new" 121st Engineer Battalion was established in the Maryland Army National Guard. Although it carries the same name as the World War II unit, the new MDNG unit has NO previous history.
On 6 June 1944, the 121st Engineer Battalion landed on Omaha beach in Normandy with the first American forces. The company endured much damage to equipment and soldiers, but after some recovery, it continued to assist in the invasion. The division was given several awards for their actions during the invasion.
The 121st Engineer Combat Battalion, less Company A, with the 112th Engineer Combat Battalion attached, landed on Omaha Beach as part of the 116th Infantry Combat Team, which was part of the First Infantry Division Landing Team. The first units of this Battalion to land were two platoons of Company "B" with Lieutenant Colonel Ploger, the Battalion Commander, and a small staff, from two LCM’s at 060710 B June 1944. These platoons were closely followed by an LCM containing one platoon of "B" Company and one platoon of "C" Company landing ten minutes later. Advance element of Battalion Headquarters landed from an LCL(L) at 060730 B. The landings were made under heavy mortar, artillery, and machine gun fire in as much as no infantry had preceded the landing of the Engineers on Dog Green and part of Dog White beach. A direct artillery hit on the bow of the LCI(L) was made just as unloading began and many engineers became casualties as a result of the blast and following fire. It is estimated that 50% of this initial force were casualties, and 75% of the equipment was lost. Captain Holmstrup, Commanding Officer, Company "C", was killed as he left the landing craft. Our initial effort was to regroup our forces and to gather enough material on the beach to accomplish our missions. Some elements of the Battalion proceeded immediately inland and entered Vierville-sur-Mer at about 1000 B hours, cross country by way of the bluff overlooking the beach east of Vierville.
Two platoons of "C" Company landed on Easy Green beach from LCT’s at 061030 B hours with bulldozers and about one ton of explosives each. Remnants of Company "B" and Company "C" were then directed to exit D-1 to open it for traffic. It was necessary to wipe out several sniper positions before actual work could begin. Approximately 30 prisoners were taken in the ensuing action. At about 1200 B hours an officer patrol, formed in Vierville-sur-Mer and led by General Cota, Assistant Division Commander and containing Major Olson, Engineer Battalion Executive, Captain Bainbridge, ADE of 254th Engineers, Lieutenant MacAllister, Engineer Battalion Adjutant, and others, proceeded through the D-1 Exit from the rear before it was opened, rejoining the elements of the Battalion at work on the beach and beach exit. The wall blocking the beach exit D-1 was breached with an external charge of 1100 lbs of TNT. The resultant vehicles. Company "B" remained on the beach to complete opening of the beach exit ant to clear the road to Vierville-sur-Mer. These missions were accomplished by 062100 B hours. Meanwhile, Company "C" was ordered to clear transit area #1 west of Vierville-sur-Mer, which mission was about 50% completed that night. Company "C" bivouacked in transit area # 1, "H&S" and "B" Company bivouacked just south of Vierville. At 070530 B June 1944 the enemy attacked our positions from the south. The elements of the Battalion withstood the attack until the arrival of Rangers supported by tanks at about 1030 B hours. We then set up a defensive line for protection of Division CP along the east-west road through Vierville. Captain Humphrey, Commanding Officer, Company "B", was wounded while leading a part of Company "B" out the assembly area. At 071600 B hours Company "C" continued with their mission of cleaning transit area # 1, which was completed at 071900 B hours. Company "A" landed with the 115th Combat Team, the leading platoon landing at 062300 B hours. Company "A" supported for the forward movements of the 115th CT by opening lines of communication behind their leading elements. While passing through Saint-Laurent-sur-Mer, Company "A" came under fire of some snipers, at which time Captain Martin, the Commanding Officer, was wounded and evacuated. Company "A" joined the Battalion the evening of 7 June. Company "C" on the morning of 8 June cleared the town of Vierville-sur-Mer of rubble caused by artillery bombardment during the previous evening. Company "A" cleared the road leading to Grandcamp on 8 June.
The 112th Engineer Combat Battalion, attached to the 116th CT for the landing, reverted to unit control on 7 June 1944.The Advance to and Crossing of inundated area, Capture of Isigny, and the Advance to the River Elle. Each Infantry regiment in this phase of operations was supported by engineers who cleared roads to their forward elements. As soon as Isigny was captured the C Company bulldozer was dispatched there and cleared the road through the town which had been severely damaged by aerial and artillery bombardment. Company A supported the 115th Infantry in crossing the inundated area by building ten improvised foot bridges using assault raft equipment, pneumatic floats and bridge timbers. The 254th Engineers supporting this Battalion constructed four short treadway bridges on the road from La Cambe to Douet, and also constructed a 40-foot double single Bailey Bridge at 473847 on the Isigny-Carentan road. Our Battalion continued to support the advance of the infantry south of inundated area to the River Elle with clearance and maintenance of routes of communication. Our first water point was established at 610907 and opened at 1900B hours on 9 June 1944.
At the end of World War II the Battalion was deactivated. It was reactivated as part of its original organization, the District of Columbia Army National Guard, as the 163rd Military Police Battalion and its history, lineage and honors continue today in the DCARNG's 372nd Military Police Battalion.
In 1948, a new "121st Engineer Battalion" was activated in Maryland as part of the Maryland National Guard. This unit carries the designation of the old 121st Engineers, but has no historical link to that organization. The new battalion played a pivotal role in the crowd control efforts after being called in to assist the local authorities during the race riots that took place in Baltimore and Cambridge in the 1960s.