. At the start of the Civil War, McCook received a commission as
colonel of the
1st Ohio Infantry in April 1861. He served in the Washington defenses and saw action at the
First Battle of Bull Run. McCook was promoted to captain in the regular army in May. On September 3, 1861, he was promoted to
brigadier general of volunteers, commanded a brigade in Kentucky that fall, and led a division by February 1862. He earned the
brevet of
lieutenant colonel in the
regular army for his part in the capture of
Nashville, Tennessee. McCook then commanded the 2nd Division in the
Army of the Ohio at the
Battle of Shiloh on the second day of fighting, and then in the subsequent
campaign against Corinth. He was promoted to
major general of volunteers on July 17, 1862. McCook was given command of the
I Corps in the Army of the Ohio. His corps suffered heavy casualties and was driven back a mile at the
Battle of Perryville in October 1862. Command of the Army of the Ohio was reorganized, and his command was designated the Right Wing of the
XIV Corps in the new
Army of the Cumberland. His command again suffered heavy losses at the
Battle of Stones River. Once again, the command structure was reorganized, and his corps was named the
XX Corps. For the third and final time, at
Chickamauga, McCook's troops suffered heavily and were driven from the field. He was court-martialed and partially blamed for the Union disaster at Chickamauga. He was not convicted but relieved of duty in the Army of the Cumberland. McCook's demotion from command was in large part political, as he was a pro-slavery Democrat and had earned the ire of Secretary of War
Edwin Stanton. He waited almost a year before receiving another command assignment of any kind. It came thanks in part to Confederate General
Jubal Early and his threat against Washington, D.C. McCook was placed in command of the "Defenses of the Potomac River and Washington" and was in charge of all forces defending the capital at the
Battle of Fort Stevens. The day the battle ended, so did McCook's command of the city's defenses, and he was again without command. At the close of the war, he was given command of the District of Eastern Arkansas. He received brevet promotions to brigadier general and major general in the regular army for service throughout the war. ==Postbellum career==