I think it was Don Carlos Buell, who was fired for his sluggish pursuit of Bragg in Kentucky, and replaced by William Rosecrans.
below Nashville, Tennessee [:
George B. McClenllan.
General Ulyesses S. Grant
George Thomas
General order #168, October 24, 1862
Robert E. Lee
In 1863, Major General Alexander M. McCook was the commander of the XX Corps of the Army of the Cumberland. He was a USMA at West Point graduate who graduated in 1852. He was a veteran of the Indian wars and later was an instructor at the USMA at West Point. Before joining the Army of the Cumberland, McCook took part in the First Battle of Bull Run. He was a division commander at the Battle of Shiloh, and fought at Perryville and Stone's River. His commander in the Army of the Cumberland, Major General William Rosencrans saw weakness in McCook as a battlefield commander.
The Confederate Army of Tennessee was formed on November 20, 1862. It was encamped near Murfeesboro, TN. After an inconclusive battle against Union General Rosecrans, the Army moved down to Tullahoma, TN.
Major General Irvin McDowell actually had two stints as commander of the Union Army. The first was from March 13, 1862 until April 4, 1862. The second was from June 26, 1862 _ September 5, 1862.
The Commander in Chief of the Confederacy was his President, Jefferson Davis. Robert E. Lee became the commander of the Army of the Northern Virginia in 1862 and General in Chief of the Confederate Army on February 6,1865.
In June of 1862, General PT Beauregard was replaced by General Braxton Bragg as the new commander of the Army of Mississippi. Bragg then formulated plans for a future invasion into the Border State of Kentucky.
Major General Thomas L. Crittenden was the commander of the XXI Corps in the Army of the Cumberland in 1863. Crittenden was not a USMA graduate.He was a lawyer until the outbreak of the Mexican American War. He enlisted in the army and reached the rank of colonel. He sided with the Union when the US Civil War broke out. He was from Kentucky but was a Unionist. He was given command in the pro-Union Kentucky militia. He served in the battles of Shiloh and Corinth. He served first under General Buell and when that army transitioned into the Army of the Cumberland, The commander of the Army of the Cumberland in 1863 was Major General Rosencrans. Rosencrans had doubts about the abilities of Crittenden because he was not a USMA graduate. Nevertheless, Crittenden impressed Rosencrans for his bravery and gave him important assignments in the Tullahoma campaign.