The 3rd Infantry saw extensive service during the American Civil War and was credited with 12 campaigns. Detachments from the regiment were serving at Fort Pickens in Florida and in Saluria on the Gulf Coast of Texas when the war began in April 1861. Three companies of the 3rd Infantry surrendered on 25 April. Five of the regiment's 10 companies were engaged at the Battle of Bull Run on 20 July 1861.