In 1861, at the outbreak of the American Civil War, Twitchell enlisted with the Fourth Regiment of Vermont Volunteers and fought in fourteen battles.

A history of the Tenth Regiment, Vermont Volunteers, Rutland, Vt.: Tuttle Co., 1894 (2nd edition).

The suburb of Vermont is the location of the Vermont Volunteers Roll of Honour for the First World War.

He was educated at Yale University, and seved in Company D of the 16th Vermont Volunteers during the American Civil War, reaching the rank of corporal.

He attended Bakersfield and Barre Academies and served in the Union Army during the Civil War in the Company A, 3rd Vermont Volunteers, enlisting below the age of 20.

He returned to Vermont and served during the Civil War in the Union Army as major in the Eighth Regiment, Vermont Volunteers.

D, 10th Vermont Volunteers.

Original members mustered out on September 20, 1864, and recruits were reorganized as the 1st Company Heavy Artillery, Vermont Volunteers, on March 1, 1865.

The left of the Union line lay near the river, while the right was in the woods and was commanded by 1st Lieutenant T. S. Peck, Company H, 9th Vermont Volunteers.

When the American Civil War broke out, he served in the Union Army as major and lieutenant colonel of the Second Vermont Volunteers.