At the beginning of the American Civil War in 1861, Wistar chose to follow his home state and the
Union cause. He raised a
company of men and was elected its
captain. Wistar's company was added to the
71st Pennsylvania Infantry, originally known as the
California Regiment. On June 28, Wistar was promoted to the rank of
lieutenant colonel, Beginning on July 18, 1863, Wistar commanded the District of Yorktown in Virginia, and that August the post was re-designated as a subdistrict of the Department of
Virginia and
North Carolina. In April 1864, he briefly was given
divisional command of the
XVIII in the
Army of the James. On May 7, Wistar resumed leading a brigade and participated in the
Bermuda Hundred Campaign, but 11 days later he was relieved of duty and replaced by Col. Griffen Stedman. Military historian
Ezra J. Warner surmises Wistar performed poorly during this campaign: Wistar's resignation from the Union Army was accepted by the
U.S. War Department on September 15, 1864. ==Postbellum career==