At the outbreak of the war, he was appointed to the rank of major in the First Rhode Island Detached Militia, a "90-day" regiment on May 2, 1861. Balch succeeded to command of the regiment when its previous commander, Colonel Ambrose Burnside was promoted to brigadier general and given command of a brigade. Balch commanded the 1st Rhode Island at the First Battle of Bull Run on July 21, 1861.