On July 29, 1862, Frick was honorably discharged in order to assume command of a new regiment, the 129th Pennsylvania Infantry. Commissioned as a colonel, he re-enrolled and re-mustered for military service at Camp Curtin in Harrisburg, Pennsylvania on August 15. Transported with his men to Washington, D.C., he and his regiment then made camp near Alexandria, Virginia, on August 18. Assigned to guard duties there, two of his companies were then detached and assigned to build a bridge across Bull Run. The remainder of his men were then ordered to guard a Union ammunition train on August 30 near Centreville, where they came under brief, but heavy enemy artillery fire. Attached to the brigade commanded by Union General E. B. Tyler beginning September 3, he continued to drill his men at their encampments at Fairfax Seminary and Fort Richardson. He and his men experienced their first true exposure to combat in December during the Battle of Fredericksburg. According to historian Samuel Bates: