The regiment was mustered into Federal service on July 12, 1861, and left for the seat of war, arriving along the Potomac on August 25. On October 21, it was engaged with the heaviest loss among all Federal regiments at the Battle of Ball's Bluff. In the spring of 1862, it was made a part of the II Corps of the Army of the Potomac and accompanied it during the Peninsular Campaign, being engaged at the Battles of Seven Pines, Savage's Station, and Glendale with modest losses. In April 1862, the 1st Company Massachusetts Sharpshooters was attached to the regiment, serving with it until the spring of 1863. Spending most of the summer at Harrison's Landing, it departed in August just in time to miss the Battle of Second Bull Run. The regiment then embarked upon the Maryland Campaign, where it was savagely flanked by the Confederates at the Battle of Antietam, losing over 50% of its 600 men. During the Battle of Fredericksburg, it was kept out of the main assault on Marye's Heights and suffered relatively few losses. The Battle of Gettysburg found the regiment engaged against the assaults of the Army of Northern Virginia on July 2 and 3, 1863, with heavy loss. By 1864, the regiment's strength had dwindled, but it still faced rigorous campaigning with action at The Battle of the Wilderness, Spotsylvania, Cold Harbor, and the Siege of Petersburg. By June 22, the regiment could field only 75 officers and men. This small group was captured en masse that day when they were outflanked by a Confederate force on the Jerusalem Plank Road. The survivors and parolees mustered out of Federal service on July 26, 1864, with its recruits and re-enlistees being transferred to the 20th Massachusetts Volunteer Infantry.
Casualties
During the war, the 15th Massachusetts Regiment sustained the 10th-highest number of men killed or fatally wounded in action among all 1,200 Federal regiments. Its losses at several engagements are as follows:
Ball's Bluff, Va., October 21, 1861: 40 killed or fatally wounded, 70 wounded, 202 captured
Antietam, Md., September 17, 1862: 104 killed or fatally wounded, 206 wounded, 14 captured
Gettysburg, Pa., July 2–3, 1863: 37 killed or fatally wounded, 94 wounded, 24 captured
Wilderness, Va., May 5–6, 1864: 16 killed or fatally wounded, 51 wounded, 17 captured
In sum, 14 officers and 227 enlisted men were killed or fatally wounded during the course of the war.
