Army of the Mountain Department (USA)

March 11–June 26, 1862

The Army of the Mountain Department was the unofficial designation for a force of roughly 25,000 Union soldiers commanded by Major General John C. Frémont during the Shenandoah Valley Campaign of 1862.

General John C. Fremont, Civil War, USA

Major General John C. Fremont was the first commander of the Army of the Gulf. [Wikimedia Commons]


On November 1, 1861, the U. S. War Department issued General Orders, No. 94, announcing the retirement of General Winfield Scott and an executive order from President Abraham Lincoln proclaiming that “Major-General George B. McClellan . . . [would] assume the command of the Army of the United States.” Lincoln hoped that under the leadership of George B. McClellan, federal forces would quickly end the Southern rebellion.

McClellan dashed Lincoln’s hopes, however, as he spent the next five months reorganizing his forces and preparing for a roundabout assault on the Confederate capital at Richmond, Virginia, instead of the direct and decisive blow that Lincoln desired.

Despite his early infatuation and support for McClellan, Lincoln eventually tired of the general’s insolence and indecisiveness. When McClellan refused to share plans for his grand assault on Richmond, Lincoln began acting as his own general, convening private war councils with McClellan’s subordinates and making his own plans.

By the spring of 1862, Lincoln lost his patience. On March 11, he issued “President’s War Order, No. 3” relieving McClellan of all commands except the Department of the Potomac. Lincoln also ordered the creation of a new military department — the Mountain Department — encompassing “the country west of the Department of the Potomac and east of the Department of the Mississippi.” The President appointed Major General John C. Frémont to command the new department, which included parts of six states — Virginia, Maryland, Tennessee, Kentucky, Ohio, and Michigan. Upon assuming command of the new department on March 29, 1862, Frémont began referring to the men under his command as the Army of the Mountain Department.

Shenandoah Valley Campaign of 1862

Prior to the formation of the Army of the Mountain Department, Major General Nathaniel Banks, Union commander of the Department of the Shenandoah, led much of the 5th Corps of the Army of the Potomac into Virginia near Harpers Ferry. Banks intended to move south (up the Shenandoah Valley) to support McClellan’s planned offensive against Richmond. Opposing Banks was the Confederate Army of the Valley, a small force of roughly 4,200 soldiers commanded by General Thomas “Stonewall” Jackson.

Union Victory at Kernstown

Events went well for the Yankees initially. Faced with the overwhelming size of Banks’ force, Jackson abandoned his headquarters at Winchester, Virginia, on March 11, 1862. On March 23, Union forces commanded by Brigadier General James Shields and Colonel Nathan Kimball defeated Jackson’s Division at the First Battle of Kernstown.

Following the Rebel defeat at Kernstown — the only loss of Jackson’s career as a commanding officer — the Confederate general retreated south to the central valley, but Banks chose not to pursue him. Jackson spent the next several weeks reinforcing and reorganizing his forces. In mid-April, Confederate officials dispatched Major General Richard Ewell’s division to the Shenandoah Valley, increasing the size of Jackson’s command by 8,500 soldiers.

Frémont’s Role

On March 29, 1862, John C. Frémont assumed command of the Mountain Department at Wheeling, Virginia (now West Virginia). Frémont quickly marched the roughly 25,000 men he commanded toward the Shenandoah Valley. Washington officials instructed Frémont to work in conjunction with Banks to thwart Confederate operations in the Shenandoah Valley and prevent Confederate forces from reinforcing Richmond.

Battle of McDowell — May 8, 1862

By early May, Union forces were closing in on Jackson’s reinforced army. Frémont’s Army of the Mountain Department was approaching from the west, and Banks’ forces were converging from the east. In danger of being caught in a pincer movement, Jackson resolved to defeat each army separately before they could unite against him.

In early May, Jackson marched 10,000 soldiers west to confront two brigades of Frémont’s command, led by Brigadier General Robert H. Milroy and Brigadier General Robert C. Schenck. On May 8, as Jackson neared the village of McDowell, Milroy launched an unexpected attack upon the Confederates. The Federals initially surprised Jackson’s men, but the Rebels held their ground and repulsed the Yankees. After four hours of heavy fighting, Milroy and Schenck melted back into the mountains west of the Valley. The Confederate victory at the Battle of McDowell prevented Frémont from joining forces with Banks to destroy Jackson’s army.

Lincoln’s New Plan

Following the Confederate victory at McDowell, Jackson turned his attention to Banks’ forces, He dealt the Federals significant blows at the Battle of Front Royal (May 23, 1862) and the First Battle of Winchester (May 25, 1862).

Jackson’s victory at the First Battle of Winchester created a great deal of angst in Washington, especially with President Lincoln. Weary of Federal defeats in the Shenandoah Valley, Lincoln devised his own complicated plan to stop Jackson’s string of victories. The president ordered Frémont to re-enter the Valley from the west near Harrisonburg and to disrupt Jackson’s supply lines.

Frémont’s movement was part of a larger plan to trap Jackson between three federal forces, but bad weather and poor roads delayed Frémont’s advance. Jackson’s cavalry skirmished with Frémont’s troops as they advanced into the Shenandoah Valley during the first week of June.

Battle of Cross Keys — June 8, 1862

On June 8, Jackson divided his army and sent Ewell’s Division west across the South Fork of the Shenandoah River near Cross Keys. Ewell’s orders were to hold off Frémont, while Jackson moved against other Union forces to the northeast. After Ewell crossed the river, Union Brigadier General Julius Stahel ordered his brigade to attack the Confederate right. In doing so, Stahel exposed his men to a withering fire from Brigadier General Isaac Trimble’s brigade, and the assault failed. Later in the day, Jackson sent Richard Taylor’s brigade to reinforce Ewell, and the Confederates drove Frémont back. Despite enjoying a two-to-one numerical advantage, Frémont settled for an artillery engagement while cautiously probing the center of the Confederate line before withdrawing.

Dissolution — June 26, 1862

The Confederate victory at the Battle of Cross Keys heightened President Abraham Lincoln’s frustration with the uncoordinated Federal setbacks in the Shenandoah Valley. On June 26, 1862, the President issued an executive order declaring that “The forces under Major-Generals Frémont, Banks, and McDowell, including the troops now under Brigadier-General Sturgis at Washington, shall be consolidated and form one army, to be called the Army of Virginia.” Lincoln further stipulated that “The troops of the Mountain Department, heretofore under command of General Frémont, shall constitute the First Army Corps, under the command of General Frémont.”


After being folded into the Army of Virginia as the First Army Corps, the men of the Army of the Mountain Department took part in the Union defeat at the Second Battle of Bull Run (August 29–30, 1862).

On September 12, 1862, the U.S.War Department issued General Orders, No. 129, ending the existence of the Army of Virginia. The order also re-designated the First Army Corps as the Eleventh Army Corps. The Eleventh Corps later fought with distinction at the Battle of Chancellorsville (April 30–May 6, 1863) and the Battle of Gettysburg (July 1–4, 1863).

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  • Article Title Army of the Mountain Department (USA)
  • Date March 11–June 26, 1862
  • Author
  • Keywords Army of the Mountain Department
  • Website Name American History Central
  • Access Date May 22, 2024
  • Publisher R.Squared Communications, LLC
  • Original Published Date
  • Date of Last Update January 12, 2024