After McClellan's failure at Antietam, Lincoln was insistent that Lee be pursued and captured. For failing to do so, McClellan was fired and Ambrose Burnside replaced him.
Burnside's order from Lincoln was to be more aggressive.
Lee was aware of Lincoln's aggressive objective given to Burnside
Lee was rather pleased by Lincoln's orders; it allowed him to assume a defensive posture on his home turf.
CSA DECISION: You are Lee. You know that General Burnside is coming into your state to attack you. How do you exploit this? What kind of tactics can you use defensively that give you the advantage?
USA DECISION: You are Burnside. Your advisor, General Hancock, believes that time is of the essence to prevent Lee from building defenses. Hancock wants you to attack Lee before all of your army and pontoon bridges are prepared. Do you hurry and attack or wait until your whole army is ready?
Click here to see what Lee and Burnside both did