In the spring of 1861, Union forces moved into northwestern Virginia to secure the Baltimore and Ohio Railroad, protect important turnpikes, and support Unionists against Confederates, fighting numerous engagements between June and December, including Philippi, Rich Mountain, Corricks Ford, Cheat Summit Fort, Carnifex Ferry, and Camp Allegheny. These Union victories enhanced Gen. George B. McClellan's reputation and damaged that of Confederate Gen. Robert E. Lee, although that situation reversed in 1862, and despite later Confederate raids, what is now West Virginia remained largely under Federal control for the rest of the war. After McClellan left for Washington in July 1861, Gen. Joseph J. Reynolds took command at Huttonsville and ordered fortifications built on the main roads leading south. By July 30, the 15th Indiana Infantry had encamped along the Tygart Valley River on the Huntersville road seven miles south of Huttonsville and began fortifying the narrow valley floor at Elkwater. The 3rd Ohio Infantry arrived on August 4 and built earthworks on a bluff above the river, naming the seven-sided redoubt Fort Marrow for their colonel, Isaac H. Marrow. As the key to the Union defensive position at Camp Elkwater, Fort Marrow commanded the approach to the main camp, allowed artillery to fire on approaching forces before they came within small-arms range, and could support infantry in the forward works. Well sited on a hill above the turnpike and overlooking a ravine leading into camp from the west, it would have been difficult for infantry to assault. Camp Elkwater was abandoned by 1862, though raiding parties from both sides later bivouacked there during the war.