During the American Civil War Freitag enlisted in the Fourteenth Regiment, (also known as the 84th New York Volunteer infantry), in 1862, at the age of 19 in Brooklyn, New York. He served as a private in Company B of the 14th Brooklyn and was promoted to corporal in 1863. On May 8, 1864, he was seriously wounded at the Battle of Spotsylvania, Laurel Hill, Virginia. He was honorably discharged in June 1865. He returned to convalesce in New York where he painted a scene of the Spotsylvania battle, which is when his art career began.