Constituted in May 1802 as the Columbian Brigade of the District of Columbia Militia, the antecedents of the 121st Engineers served with distinction as infantry in the War of 1812, Mexican War, Civil War, war with Spain, and World War I. In January 1921, various DC National Guard infantry units consolidated and converted to engineers. The unit was designated the 121st Engineer Regiment in 1922 and assigned to the 29th Division. Federalized on 3 February 1941, the regiment moved to Ft. Meade, Maryland, to train, then divided into two separate battalions when the division reorganized in March 1942. Only the 1st Battalion, redesignated the 121st Engineer Battalion, remained in the 29th as the divisional engineers. The 121st Engineer Combat Battalion embarked on HMS Queen Elizabeth for Britain in October 1942 and moved to Tidworth Barracks in Wiltshire. In May 1943, the battalion shifted to Paignton, Devonshire, to begin amphibious assault training. In September 1943, its component companies relocated to Cornwall to support the 29th Division's three regimental combat teams, with Company A attached to the 115th, Company B to the 116th, and Company C to the 175th. The battalion moved into marshalling areas on 15 May 1944 and received orders for the D-Day invasion. Less Company A, it was attached to the 116th Infantry for the initial landing to support first-wave assault troops on the western sector of Omaha Beach and to clear the D-1 Draw at Vierville. Two platoons of Company B, led by battalion commander Lt. Col. Robert Ploger, landed between D-1 and D-3 Draws at 0710, and elements of Company C followed. Despite heavy losses from artillery and machine gun fire, engineer teams accompanied infantrymen up the steep coastal bluff, penetrated inland, and helped secure Vierville at 1000. At the D-1 Draw, a 100-foot-long concrete wall blocked the exit road. Around noon, Ploger and Brig. Gen. Norman Cota ordered several bulldozers, each with a ton of explosives, to move west to the draw. Though hindered by sniper fire, about twenty engineers hauled the explosives to the base of the wall and, directed by Sgt. Noel Dube of Company C, opened a fifteen-foot gap with 1,100 pounds of TNT. Under fire, engineer bulldozers cleared the debris, and by 2100 vehicles could use the exit road from the beach to Vierville. Starting at 1030 on the eastern sector of Omaha Beach, Company A accompanied the 115th Infantry ashore, supported the regiment's advance up the bluff between E-1 and E-3 Draws, and entered St. Laurent sur-Mer that afternoon. At 0530 on D+1, the enemy threatened the tenuous beachhead in the western sector by launching a counterattack against the battalion south of Vierville. After hours of intense fighting, the battalion repulsed the enemy. The first two days of combat in Normandy cost the battalion 98 casualties, including 27 killed. The U.S. Army awarded the 121st Engineer Combat Battalion the Presidential Unit Citation for its pivotal role on D-Day, and the citation concluded that its outstanding performance of duty was a substantial contribution to the success of the assault operation.