Lowell H. Harrison, "Buell, Don Carlos," American National Biography Online, February 2000, http://www.anb.org/articles/05/05-00099.html.
In the late summer [of 1862}, when Edmund Kirby Smith and [Braxton] Bragg invaded Kentucky, [Don Carlos] Buell left a holding force in Nashville but followed Bragg with most of his army and secured Louisville before the Confederates could occupy it. Then, moving with unusual rapidity, Buell launched a multipronged attack against the scattered Confederates. At Perryville on 8 October, neither side employed all of its strength. The Confederates made gains in savage fighting but then withdrew from the state. Unable to bring the Confederates to another battle, Buell soon gave up the pursuit. President Abraham Lincoln and Secretary of War Edwin M. Stanton blamed Buell for not forcing a decisive battle and crushing the Confederate forces. Buell's belief that a conciliatory policy would hasten the reconstruction of the Union displeased the Radicals, who were demanding total war with significant changes in the postwar South. At least in part, Buell was sacrificed to the imminent November elections. Removed from command on 24 October 1862, he was replaced by William S. Rosecrans. In late 1862-early 1863 Buell ably defended his conduct before a commission that included several hostile critics, and no recommendations were made as to his future. Although Grant recommended that he be reinstated, Buell did not get another assignment, and on 23 May 1864 he was separated from the volunteer army….His talents for discipline, logistics, and administration did not fully compensate for his perceived deficiencies in aggressive and decisive leadership.