The Second Infantry was first organized under the call for volunteers for three months service. The full number of troops under this call having been accepted, the Second was mustered, June 11, 1861, into the service of the United States for "Three years or during the war" under the call of the President, dated May 3, 1861, for "500,000 men."
The organization left the state on June 20, and arrived in Washington, D.C. June 25, 1861. From this date until the regiment was mustered out, the Second Infantry served in that part of the Union Army known as the Army of Virginia and the "Army of the Potomac. "In July 1861 the Second was brigaded with three New York regiments of Infantry, the whole under command of Colonel, and later General, William T. Sherman, and in this organization participated in the battle of Bull Run, Va. July 21, 1861. On the 27th of August 1861, the regiment was assigned to the brigade under the command of Brig. Gen. Rufus King, which brigade became subsequently known as the "Iron Brigade. "With that brigade the Second participated in all the historic battles fought by the Army of the Potomac until May 11, 1864. After the battle of Laurel Hill the Second having become reduced to less than 100 men present for duty, was permanently detached from the brigade and was thereafter assigned to duty as provost guard of the Fourth Division of the Fifth Army Corps, of which the Iron Brigade was at that time a part, and following the march of that division crossed the North Anna River on the 21st, arriving on the 6th of June on the Chickahominy, VA, where the regiment remained until the expiration of its term of service.
On the morning of June 11, 1864, the Second started for Wisconsin, except those members who had re-enlisted, and recruits whose term of service had not expired. The Second reached Madison on the 18th of June and was mustered out of the military service of the United States on the 2nd day of July 1864.