At the start of the Civil War on April 19, 1861, "Gaithers Raiders", part of the Confederate "Howard County Dragoons" from Oakland Manor, marched through Ellicott's Mills to Baltimore, responding to the Baltimore riot of 1861, before heading south to join J. E. B. Stuart. Later that month, Union Army troops seized the "Winans Steam Gun" which had been en route to Harpers Ferry, Virginia, at Ellicott's Mills. The experimental gun had been developed by local Southern-sympathizer railroad builder and industrialist Ross Winans. In the fall of 1862, the 12th New Jersey Volunteer Infantry was assigned to guard Ellicott's Mills, setting up the 1,200-man Camp Johnson on the lawn of the nearby Patapsco Female Institute. On July 10, 1864, the third Confederate invasion of the North, led by General Jubal Early, forced the retreat of the Federal troops under the command of General Lew Wallace down the National Pike from the Battle of the Monocacy to the B.&amp; O.'s Ellicott's Mills station and to Baltimore. The one-day delay by Wallace's small force at Monocacy Junction enabled Lt. Gen. Ulysses S. Grant to rush troops in time to defend the U.S. capital. Homes and churches in Ellicott's Mills were temporarily used as hospitals for the Union wounded.