After the outbreak of the Civil War in early 1861, Fessenden spent the summer recruiting and organizing a company of skilled riflemen and marksmen that became a company in the United States Sharpshooters. In November of that year, he received a commission as a
captain in the sharpshooter company and served in the defenses of
Washington, D.C. Through the influences of his powerful father, he received a promotion to
lieutenant colonel and a new position as a staff officer and
aide-de-camp to Union
Maj. Gen. David Hunter. He held that position from March 1862 until January 1863, serving primarily in the Department of the South and the
X Corps. During that period, he was promoted to
colonel in July 1862. He was involved in planning the Union operations against the
Confederate defenses of
Charleston, South Carolina, but missed participating in the major attacks while recovering in Washington, D.C., from a riding accident suffered in the field. During his recuperation, he frequently used his oratory and organizational skills to recruit new volunteers for military service. In November 1863, Fessenden became an aide-de-camp to Maj. Gen.
Joseph Hooker, who had been sent to
Tennessee with two
corps of the
Army of the Potomac to reinforce the
Army of the Cumberland. Hooker commended him for his bravery at the
Battle of Missionary Ridge and recommended his promotion to brigadier general. He was commended three times for his actions during the
Atlanta campaign and again recommended for higher rank by Hooker. In August 1864, he finally received his long desired promotion to brigadier general, but did not get a field command initially as his sponsor Hooker had since left the army. He was assigned to command a brigade in the
Army of the Shenandoah in the
Shenandoah Valley region of
Virginia, an assignment initially meant for his brother, who lost a leg in the
Red River Campaign that precluded his taking the assignment. James Fessenden arrived in Virginia and assumed command of a brigade in the
XIX Corps under Maj. Gen.
William H. Emory. He participated in the
Battle of Cedar Creek in October, engaged in scouting duty, and then was assigned command of the Union garrison in the defenses of
Winchester, Virginia, in January 1865. In May 1865, he led his brigade in the
Grand Review of the Armies in Washington, D.C., following the surrender of the two leading Confederate armies in the
Eastern Theater. He subsequently commanded troops under Maj. Gen.
Winfield S. Hancock and an occupation garrison in South Carolina before mustering out of the army on January 15, 1866. He received a
brevet promotion to major general before leaving the service. ==Postbellum career==