Between September 4th and 7th, 1862, the Army of Northern Virginia under General Robert E. Lee crossed the Potomac near Leesburg and occupied Frederick, Maryland. On the 10th, a movement began to surround and capture the Union forces at Harper's Ferry. Early that morning, Major-General T. J. Jackson, with Jackson's (Stonewall) Division and the divisions of R.S. Ewell and A.P Hill, left Frederick, marched over South Mountain at this pass, crossed the Potomac near Williamsport on the 11th, seized Martinsburg on the 12th, and by way of Charlestown invested Harper's Ferry from the Virginia side of the Potomac on the 13th. J.G. Walker's Division, then near Monocacy Aqueduct, recrossed the Potomac at Point of Rocks on the night of the 10th and occupied Loudon Heights on the 13th. Major-General Lafayette McLaws, with his own division and R.H. Anderson's, both of Longstreet's command, moved from Frederick on the 10th by way of Middletown, crossed South Mountain at Brownsville Pass on the 11th, and on the 12th sent two brigades to Maryland Heights and six down Pleasant Valley, investing Harper's Ferry from the Maryland side. Generals Lee and Longstreet, with the divisions of D.R. Jones and J.B. Hood, the brigade of N.G. Evans, and the Reserve Artillery, marched on this road to Hagerstown. D.H. Hill's Division halted at Boonsboro to prevent the escape of the garrison at Harper's Ferry through Pleasant Valley and to support Stuart's Cavalry, which remained east of South Mountain to observe the movements of the Union Army and retard its advance.