mccook was given command of the i corps in the army of the ohio. his corps suffered heavy casualties and driven back a mile at the battle of perryville in october 1862. command of the army of the ohio was reorganized and his command 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 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 courtmartialed and partially blamed for the union disaster at chickamauga. he was not convicted but relieved of duty in the army of the cumberland.