Marston served in the
Union Army during the
Civil War. He first saw combat as
colonel of the
2nd New Hampshire Volunteer Infantry during the
First Battle of Bull Run in July 1861. His arm was shattered, but he refused an amputation. After he recovered, he fought in the
Peninsula Campaign,
Second Battle of Bull Run, and
Battle of Fredericksburg. Marston was promoted to brigadier general of U.S. volunteers, effective November 29, 1862. Prior to the
Chancellorsville campaign, he was relieved from duty with the
Army of the Potomac and assigned to the defenses of Washington where he returned to his seat in Congress. After
Gettysburg, Marston was directed to establish a prison camp in Maryland, which later became known as
Point Lookout. The area was designated the
District of Saint Mary's under the overall command of Maj. Gen.
Benjamin Butler. In 1864, he commanded a brigade in Maj. Gen.
W.F. "Baldy" Smith's
XVIII Corps during the
Bermuda Hundred Campaign. He took part in a disastrous assault on
Cold Harbor, where his brigade suffered heavy casualties. During the Union
assaults on Petersburg, Gilman assumed command of the 1st Division in the XVIII Corps on the last day of battle. Thereafter he commanded the Union troops on the north side of the James River with his headquarters located at
Fort Pocahontas. Occasionally he returned to command of the 1st Division, including a brief stint during the
Second Battle of Fair Oaks. Having been re-elected to Congress he resigned 1865, whereupon he received the thanks of the state of New Hampshire. ==U.S. Congress==