Ashby's vigorous reconnaissance and screening were factors in the success of Jackson's Valley campaign in the Shenandoah Valley in 1862. However, Ashby failed Jackson in some instances. At the First Battle of Kernstown, Jackson attacked a retreating Union column that Ashby had estimated to be four regiments of infantry, about the size of Jackson's force. It turned out to be an entire division of 9,000 men, and Jackson was forced to retreat. At the First Battle of Winchester, as Union forces under Maj. Gen. Nathaniel P. Banks were retreating, Ashby failed to cut off their retreat because his troopers were plundering captured wagons.