The regiment was transported to Washington, D.C., and saw its first combat at the First Battle of Bull Run in July 1861 in a brigade under William T. Sherman. Due to some of the men were state militia grey uniforms, the 2nd Wisconsin received some friendly fire from Union troops mistaking them for Confederates. Coon, never comfortable in his role as a military man, resigned July 30 to return to politics. Peck and the major also resigned, all were considered victims of the regiment's poor performance at Bull Run. Three new field officers replaced them—Col. Edgar O'Connor, Lt. Col. Fairchild, and Maj. Thomas S. Allen. The appointment of O'Connor, married to a Southern woman and an outspoken Democrat, was met with sharp criticism in Wisconsin's media, particularly in the Republican newspapers. He would prove to be a brave and competent leader, however, and was killed in action in Virginia in late summer 1862. Lt. Col. Fairchild was promoted to the rank of full colonel on September 8, 1862, to become the third commander of the 2nd Wisconsin Volunteers.