Short answers, pulled from the story.
Don Carlos Buell was a Union Army general who commanded the Army of the Ohio during the Civil War. He played a role at the Battle of Shiloh in April 1862 and led the Union forces at the Battle of Perryville in October 1862, after which he was relieved of command and never returned to field service.
Buell was relieved on the 24th of October, 1862, following the Battle of Perryville, where he failed to commit most of his nearly 60,000 men against roughly 16,000 Confederates and then declined to pursue Bragg's retreating army. Lincoln and Halleck had also grown frustrated with his slow movements throughout the Kentucky and Tennessee campaigns.
Buell's Army of the Ohio arrived at Pittsburg Landing on the second day of Shiloh, the 7th of April, 1862, reinforcing Grant's battered forces and helping repulse the Confederate attack. Buell refused to accept orders from Grant, insisting on independent command, and later claimed sole credit for the Union victory, a view historians including Larry Daniels and Kenneth W. Noe have disputed.
Grant defended Buell in his memoirs, calling him "a brave, intelligent officer" and saying he had requested the Secretary of War restore Buell to duty when Grant became general-in-chief in 1864. Grant also wrote that he repeatedly refuted charges of Buell's disloyalty, which he considered unjust.
Buell earned the nickname "The McClellan of the West" for his cautious approach to campaigning and his preference for a limited war that would not disrupt civilian life in the South or interfere with slavery. Like George McClellan, he consistently moved slowly and resisted political pressure to fight more aggressively.
Buell was buried in Bellefontaine Cemetery in St. Louis after his death on the 19th of November, 1898. His personal papers, maps, correspondence, uniform, and West Point diploma are held at the University of Notre Dame.