When the Civil War began in 1861, Kentucky was a state that declared its neutrality and was at risk of supporting the Confederacy. Crittenden and his father remained loyal to the Union, but his brother joined the
Confederate Army, a common occurrence in the border states. Crittenden had been a
major general in the Kentucky
militia since 1860. He was appointed
brigadier general of volunteers in September and placed in command of the 5th Division in the
Army of the Ohio. He led the division at the
Battle of Shiloh in 1862. After Shiloh, he was appointed
major general of volunteers and commanded the
II Corps in the
Army of the Ohio during the
Perryville Campaign although his corps was only lightly engaged in the fighting. When Maj. Gen.
William S. Rosecrans assumed command of the army, Crittenden's forces were redesignated the Left Wing of the
Army of the Cumberland and were heavily engaged at the
Battle of Stones River. (He received a
brevet promotion to brigadier general in the
regular army in 1867 for his service at Stones River.) The Army of the Cumberland was reorganized and Crittenden's corps was once again renamed, this time the
XXI Corps. He led the corps through the
Tullahoma Campaign and at the
Battle of Chickamauga. Crittenden and fellow corps commander
Alexander McDowell McCook were blamed for the defeat and relieved of command, but both were later exonerated and acquitted of any charges. During the
Battle of Spotsylvania in 1864, Brig. Gen.
Thomas G. Stevenson was killed leading the 1st Division, IX Corps. Stevenson's successor was Col.
Daniel Leasure. The decision was made to replace the colonel with a more experienced commander and General Crittenden was chosen to take command of the division. He assumed command on May 12 and led it during the final days of Spotsylvania and through the
Battle of Cold Harbor, before resigning on December 13, 1864. ==Postbellum career==