I'm not sure but Grant took over from Buell during/after the Battle of Shiloh. Buell led the Union forces against the rebs at the Battle of Perryville, about 50 miles from here. Tactically the Union lost because Buell was a moron, but strategically the battle convinced the Confederates that they could not hold Kentucky and they withdrew to Tennessee.
There is a ride to Shakertown that I'd like to do that passes near or through Perrysville. Some nasty hills I'm told. The Shakers themselves helped the casualties of both sides without rancor. They respected Union ideals of equality but thought the Confederates were nicer people. Go figure.