Arriving in Washington on Sept. 4, it was attached to Burnside's Corps and engaged the Lee's forces at Frederick, MD, on September 12, 1862. Company G, acting as BGEN Reno's bodyguard, took part in the Battle of South Mountain, Companies M and H, under MGEN Porter, in that of Antietam. The regiment (except Companies G, M, and H) remained at Frederick, from Sept. 12 to Nov. 2, up to which period it had lost in action and worn out in service nearly 700 horses.