In April 1861, Crittenden began his service as a captain in the 6th Indiana Infantry, although he was soon appointed colonel of this same unit. He participated in the battles of Philippi and Corrick's Ford in the campaign in western Virginia. Reassigned to Kentucky, his regiment was engaged in the second day at the Battle of Shiloh. Promoted to brigadier general, Crittenden was put in command of the post at Murfreesboro, Tennessee. The day after he took command, the post was captured by Union forces under Gen. Nathan B. Forrest. He was released in October 1862, but he saw no further service in the war due to his failure to protect his post.
