why did the battle of fredericksburg occur?

The Battle of Fredericksburg was fought December 11–15, 1862, in and around Fredericksburg, Virginia, between General Robert E. Lee's Confederate Army of Northern Virginia and the Union Army of the Potomac, commanded by Maj. Gen. Ambrose E. Burnside. It is remembered as one of the most one-sided battles of the American Civil War. The Union Army suffered terrible casualties in futile frontal assaults on December 13 against entrenched Confederate defenders on the heights behind the city, bringing an early end to their campaign against the Confederate capital of Richmond.

Background:

The battle was the result of an effort by the Union Army to regain the initiative in its struggle against Lee's smaller but more aggressive army. Burnside was appointed commander of the Army of the Potomac in November, replacing Maj. Gen. George B. McClellan. Although McClellan had stopped Lee at the Battle of Antietam in September, President Abraham Lincoln believed he lacked decisiveness, did not pursue and destroy Lee's army in Maryland, and wasted excessive time reorganizing and re-equipping his army following major battles.

Burnside, in response to prodding from Lincoln and General-in-Chief Maj. Gen. Henry W. Halleck, planned a late fall offensive; he communicated his plan to Halleck on November 9. The plan relied on quick movement and deceit. He would concentrate his army in a visible fashion near Warrenton, feigning a movement on Culpeper Court House, Orange Court House, or Gordonsville. Then he would rapidly shift his army southeast and cross the Rappahannock River to Fredericksburg, hoping that Robert E. Lee would sit still, unclear as to Burnside's intentions, while the Union Army made a rapid movement against Richmond, south along the Richmond, Fredericksburg and Potomac Railroad from Fredericksburg. Burnside selected this plan because he was concerned that if he were to move directly south from Warrenton, he would be exposed to a flanking attack from Lt. Gen. Thomas J. "Stonewall" Jackson, whose corps was at that time in the Shenandoah Valley south of Winchester. He also believed that the Orange and Alexandria Railroad would be an inadequate supply line. While Burnside began assembling a supply base at Falmouth, near Fredericksburg, the Lincoln administration entertained a lengthy debate about the wisdom of his plan. Lincoln eventually approved but cautioned him to move with great speed, certainly doubting that Lee would cooperate as Burnside anticipated.