From September 24, 1860, to April 26, 1861, Breck served at West Point as assistant professor of geography, History and Ethics.

On November 29, 1861, Breck became staff captain—assistant adjutant general of General Irvin McDowell's division of the Army of the Potomac, which defended Washington, D.C., during the American Civil War or, as it was characterized at the time by the Union, "the Rebellion of the Seceding States."

In late May and early June, he was part of Union Brigadier General Irvin McDowell's unsuccessful expedition to the Shenandoah Valley to intercept the Confederate forces under General Stonewall Jackson and cut off their avenue of retreat from Winchester, Virginia.

[2] On July 2, 1862, Breck took the post of assistant in the adjutant general's office in Washington, which he held until the end of the war.

He was in charge of "Rolls, Returns, Books, Blanks and business pertaining to the enlisted men of the Regular and Volunteer Forces, and of the records of discontinued commands and the preparation and publication of the 'Volunteer Army Register.'"