The regiment left Boston July 23, 1861. It was stationed on the Maryland side of the Upper Potomac, and made its winter-quarters at Frederick, Md. It commenced active service in April, 1862, in the Shenandoah Valley, then in Hartsuff's (3d) Brigade, Ricketts's (2d) Division, McDowell's Corps. While in this command it was engaged at Manassas, where its losses amounted to 13 killed, 61 wounded, and 63 missing; Colonel Webster, a son of Daniel Webster, was killed there. The regiment faced a terrible fire at Antietam, losing 49 killed, 165 wounded, and missing, out of 334 present on the field; Major Elisha Burbank was mortally wounded in that battle. At Fredericksburg, the regiment was in Lyle's (2d) Brigade, Gibbon's (2d) Division, First Corps; its casualties in that fight were 14 killed, 86 wounded, and 4 missing, out of 258 engaged. General Baxter commanded the brigade at Gettysburg, and Robinson the division -- the regiment losing there 5 killed, 52 wounded, and 62 missing, out of about 200 in line. The division was transferred, in 1864, to the Fifth Corps. At the Wilderness, Lieutenant-Colonel David Allen, Jr., was killed, the loss of the Twelfth amounting to 14 killed, 48 wounded, and 8 missing Its losses in 1864, from May 5th to June 25th -- when it was mustered out -- were 42 killed, 124 wounded, and 10 missing. It left the front June 25, 1864; the recruits and reenlisted men remaining in the field were transferred to the Thirty-ninth Massachusetts.