The War Department marker for the United States Regular Infantry is on Boonsboro Pike across from the picnic area. (Boonesboro Pike tour map)

In the First Battle of Bull Run the small number of United States Regulars had held together and provided a disciplined shield to cover the rout of the panicked volunteer regiments. When McClellan organized the Army of the Potomac he had doubts about his volunteers and kept a hard core of U.S. Regulars together as a final reserve in case of disaster. At Antietam his unwillingness to fully commit the Regulars when Lee’s army was at the breaking point probably cost him what could have been a major victory.

From the marker:

U.S.A.

On the afternoon of September 17, the 4th U.S. Infantry, the 1st Battalion of the 12th, 1st and 2d Battalions of the 14th and detachments of the 2d and 10th U.S. Infantry, were deployed to the right and left of this point in support of the batteries on the crest in rear and to check the advance of the Confederate skirmishers. They were actively engaged until dark, when they were withdrawn to the east side of the Antietam.