Union victory at the Battle of Rich Mountain on July 11th 1861 helped rise Major General George B McClellan to command.
On June 27th 1861 Major General George B McClellan who was then in command of the Union army in western Virginia, began moving troops south from Clarksburg VA. They reached the area around Rich Mountain in Randolph County Virginia [now West Virginia] on July 9th 1861 and come up against Confederate Lieutenant Colonel John Pegram. On July 11th 1861 Union Brigadier General William S Rosecrans took the Staunton Parkersburg Turnpike to Pegram’s rear. The next two hours of fighting split the Confederates forces. Pegram surrendered on July 13th 1861. About half of his men managed to escape.
On July 22nd 1861 McClellan was ordered to return to Washington DC, where he would be place in command of the Army of the Potomac. Rosecrans would be give the command of the army in western Virginia.
Further reading on this subject
The Battle of Rich Mountain: An Overview