When the Civil War began, he joined the Confederacy and was commissioned to the rank of lieutenant in the regular Confederate army 1861.

They settled and made a family in the town, and Forsberg served as Lynchburg's city engineer for over twenty years.

He later worked as an architect in Baltimore and as a draftsman at the United States Capitol Building in Washington, D.C., at the time still under construction.

When the Civil War began, the Danish consul informed him that the United States government would offer him a commission in the army, and that if he refused it, he would be arrested.

Forsberg soon found a place on a fishing vessel that took him to Charleston, where he was employed as a volunteer topographical engineer in the defense of the city.

In August 1861, Forsberg moved to Richmond, Virginia where he met John B. Floyd, former U.S. Secretary of War, who advised him to apply to President Jefferson Davis in person for a commission in the Confederate army.

Commissioned as a lieutenant, Forsberg was detailed to serve on the staff of Floyd, then a brigadier general in command of the Confederate troops in the Kanawha Valley.

In early February 1862, the regiment, with the rest of Floyd's division, was sent to Fort Donelson, in order to strengthen its garrison.