He commanded the advance of J.E.B. Stuart's expedition to Catlett's Station, and was notable in the Second Battle of Manassas, where captured Union commander John Pope's orderly and horses. During the fighting at Crampton's Gap at the Battle of South Mountain, his cavalry delayed the advance of William B. Franklin's VI Corps with help from John Pelham's artillery. At Antietam, his men screened Robert E. Lee's left flank. He temporarily assumed command of Fitzhugh Lee's brigade during the subsequent fighting against Alfred Pleasonton.