Militia under his command played a strategic role during the Gettysburg Campaign in delaying the advance of Confederate troops of the Army of Northern Virginia and preventing their crossing the Susquehanna River, critical to Pennsylvania's defense.

[6] Couch then took a one-year leave of absence from the army from 1853 to 1854 to conduct a scientific mission for the Smithsonian Institution in northern Mexico.

[7] At the outbreak of the Civil War, Couch was appointed commander of the 7th Massachusetts Infantry on June 15, 1861, with the rank of colonel in the Union Army.

This placed the IV Corps in an isolated position, vulnerable to attack on three sides; however poorly coordinated Confederate movements allowed Couch and Casey to partially prepare entrenchments against the impending assault.

The army commander, Maj. Gen. George B. McClellan, refused to send it to the U.S. War Department, and instead Couch was promoted to major general, to date from July 4.

[10][11] On November 14, 1862, Couch was assigned command of the II Corps, and he led it during the Battle of Fredericksburg as part of Maj. Gen. Edwin V. Sumner's "Right Grand Division".

[12] Early on December 12 infantry from his corps attempted to support the Union engineers' efforts to lay pontoon bridges across the Rappahannock River and into the town.

When Confederate fire repeatedly prevented this, and a heavy artillery bombardment failed as well, the decision was made to send small groups of soldiers across in pontoon boats to dislodge the defenders.

[13] As the Union soldiers entered a smoldering Fredericksburg they began to sack the city, forcing Couch to order his provost guard to secure the bridges and collect the loot.

To better watch his men's progress Couch entered the town's courthouse and climbed its cupola, where he could see French's division advancing.

As they approached the Confederate defenses, Couch could see his men taking very heavy fire and easily repulsed, described "as if the division had simply vanished."

As other Union soldiers followed the II Corps in, Couch ordered his artillery to move into the field and blast the Confederates at close range.

When his own artillery chief protested against exposing the gun crews in this fashion, Couch stated that he agreed but it was necessary to slow the Confederate fire in some way.

After the reorganization Couch continued to lead the II Corps, with his divisions commanded by Hancock and French (both now major generals) and Brig.

In the afternoon of May 1 Hooker—normally quite aggressive—cautiously slowed his marching army, and soon he stopped their movement altogether, despite some success against the Confederates and the loud protests of his corps commanders.

Couch sent Hancock's division to bolster the Union men already engaged, and informed Hooker they could handle the enemy in front of them.

As Hancock formed his men, Couch could see Confederate artillery aiming for the massed Union columns, and he told his staff "Let us draw their fire."

The group of mounted officers clustered around a clearing where the enemy cannon could easily view them, thus attracting their fire and sparing the marching infantry; Couch and his staff also went unharmed.

The remaining corps tightened into a U-shaped formation by May 3, and Confederate artillery began shelling their positions, including Couch's men.

Assigned to protect Harrisburg from a threatened attack by Confederates under Lt. Gen. Richard S. Ewell, Couch directed militia from his department to skirmish with enemy cavalry elements at Sporting Hill, one of the war's northernmost engagements.

[23] Couch's militia then joined the pursuit of Robert E. Lee's Army of Northern Virginia into Maryland after the Battle of Gettysburg.

[24] In December, Couch returned to the front lines with an assignment to the Western Theater, where he commanded a division in the XXIII Corps of the Army of the Ohio in the Franklin-Nashville Campaign and for the remainder of the war.

Couch returned to civilian life in Taunton after the war, where he ran unsuccessfully as a Democratic candidate for Governor of Massachusetts in 1865.

[25]In 2017, General Couch's portrait was featured on a mural in Lemoyne, Pennsylvania in commemoration of the defenses mounted in the town under his name during the Gettysburg campaign.