The marker for the Twelfth Army Corps is on the north side of Mansfield Avenue near Smoketown Road. (Smoketown Road North tour map)

From the marker:

U.S.A.

Twelfth Army Corps

Maj. Gen. Jos. K. F. Mansfield, Commanding.

September 17, 1862.

The Twelfth Corps left its bivouac on the Line and Hoffman farms at daylight, moved forward, in column of Battalions in mass, and deployed in support of the First Corps, Williams’ Division on the right and Greene’s Division on the left. Its line extended from D. R. Miller’s house, on the Hagerstown Pike, in a southeasterly direction to and across the Smoketown Road. While the deployment was in progress, Maj. Gen. Mansfield was mortally wounded and carried from the field.

He was succeeded by Brig. Gen. A. S. Williams. This Corps supported, and then relieved, the First Corps.

Later in the day it formed in support of the First and Sixth Corps of the Army of the Potomac.

No. 27.