While Lee was attacking Maryland, Bragg and Buell were going at it out in the West. After the replacement of commanders on both sides, (Buell replacing Halleck and Bragg replacing Beauregard) each President ordered them to plan an offensive. Buell continued Halleck's march southward until he heard of the advancing Bragg to the east of him.
When Bragg arrived in Chattanooga he planned, with Kirby Smith, an attack on Buell that would finish him. Then Bragg and Smith would head west to attack Grant. After a month of resting Bragg began his march northward, forcing Buell to fall back to Nashville, out of his position in Decatur, Alabama. For two weeks Bragg and Buell kept moving northward until Bragg moved into the town of Munfordville where he cut the line to Louisville. Thinking that this action would cause Buell to attack him, Bragg prepared for attack, but instead Buell moved into Louisville, in order to protect the city. However, the attack never came on Louisville and Buell was forced to move for he was threatened with losing command if he did not take action. Thus, October 7, the Union soldiers moved in on on the encamped Confederates at Perryville.
The battle itself was short and started with a strong Confederate advance. They met the full 55,000 of Buell's army and were pushed back rather easily. Neither general got much, if any, credit for it. Bragg had thought he could take on a much stronger force than his own. Buell did little during the battle and didn't even pursue the enemy upon their retreat. Immediately after the battle Lincoln replaced Buell with William S. Rosecrans, hoping to, for once, find a good general in the West. Bragg was just as much a failure as Buell, but was not replaced. His men had marched over 500 miles, ended up right back were they started and had fought only one losing battle. The battle was not really considered a victory for either side, for it had stalled the movement of both armies and both armies were right back where they started.