Appointed in 1861 by Ohio Governor William Dennison, McClellan quickly rose from retired captain to major general. In the fall of 1861, General McClellan led the Union Army of Occupation into western Virginia to hold this territory for the North. After the Union loss at the First Battle of Manassas, McClellan was given command of the Army of the Potomac. In early 1862, McClellan took the offensive against the Confederate capital. He tried to come in from the coast southeast of Richmond, in what became known as the Peninsula Campaign. It was unsuccessful.
In the fall of 1862, it became his task to turn back the Confederate offensive that threatened Washington. He commanded his forces through the battles of South Mountain, and Antietam before the Confederate Army of Northern Virginia retreated back into Virginia. However, due to McClellan's reluctance to pursue Lee, Lincoln decided to remove him from command of the Army of the Potomac in November 1862. McClellan never received another military command.