In 1856 transferred to Watervliet again.
December, 1856, ordered to the Seminole Indian campaign in Florida.
First lieutenant and chief of ordnance, Department of Florida, 1857.
Assistant professor of mathematics at West Point, 1857–1861.
Expected to resign from the army to enter the ministry, but in June, 1861, accepts the colonelcy of the Third Maine Volunteer Infantry.
Commands a brigade at the battle of Bull Run.
Brigadier-general of Volunteers, September, 1861.
Loses his right arm, from two wounds, at the battle of Fair Oaks, Virginia, June, 1862.
Major-general of Volunteers, November, 1862.
Commands an army division at the battles of Antietam and Fredericksburg.