Also known as the Army of Occupation or sometimes the Provisional Army of West Virginia, the Union Army of the West was the principal field army of the Department of the Ohio, led by Maj. Gen. George B. McClellan from May 3 to July 25, 1861.
When the Union Army of the West entered Virginia on May 26, 1861, it was temporarily led by Col. Benjamin F. Kelly, commander of a regiment of ‘loyal’ Virginia troops raised in Wheeling in support of the Union. McClellan wanted his invasion to be spearheaded by these pro-Union Virginians. McClellan directed much of the first phase of his campaign via telegraph and did not take the field himself until June 22nd.
As Union regiments continued to pour into northwestern Virginia, the Army of the West expanded into two large brigades with the addition of Brig. Gen. William S. Rosecrans. Col. Kelley, who was severely wounded at Philippi on June 3rd, took a backseat and many of his regiments were strung out along the Baltimore & Ohio Railroad to protect that vital transportation route.
When Maj. Gen. George B. McClellan arrived to take command of his Army of the West in northwestern Virginia in late June, he reorganized the army and expanded it to five brigades, disbursing several of Kelley and Rosecrans’ regiments among the new units. With this accomplished, he confronted and routed the Confederate Army of the Northwest at the battles of Rich Mountain, Laurel Hill, and Corrick’s Ford. Brig. Gen. Jacob D. Cox, subordinate to McClellan, also entered Virginia through the Kanawha Valley with eight regiments.