On November 7, Maj. Gen. Ambrose E. Burnside replaced Maj. Gen. George B. McClellan as commander of the Army of the Potomac. Burnside neither wanted the command nor felt certain that he was fit for the responsibilities. Within a week, however, he had reorganized the army into 3 Grand divisions and secured President Lincoln's approval of a new drive on Richmond, Virginia.
When Burnside assumed command, the Union army covered the area of central Virginia north of the Rappahannock River near Warrenton. Across the river lay Confederate Lt. Gen. James Longstreet's I Corps, while Lt. Gen. Thomas J. "Stonewall" Jackson's II Corps remained in the Shenandoah Valley. Burnside abandoned McClellan's promising movement to strike Gen. Robert E. Lee's seperate wings for an advance eastward toward Fredericksburg.
On the 15th, the Federals began moving down the Rappahannock, uncharacteristically marching rapidly, with the advanced corps covering the 40 miles to Falmouth, across the river from Fredericksburg, in 2 days. By the 19th, the entire Union army blanketed the hills behind Falmouth. Burnside's swift, well-executed manuever surprised Lee, who temporarily lost contact with the Federals. When Lee finally ascertained Burnside's objective, he ordered Jackson east and sent Longstreet toward Fredericksburg, which his leading units reached on the 18th.
Unfortunately for the Federals, their stolen march had been for nothing. Burnside had ordered Pontoons for crossing the river, but they were sidetracked by bureaucratic bugling in the capital. he compounded the error by refusing to permit a forced crossing recommended by Maj. gen. Edwin V. Sumner, commanding the Right Grand Division. The pontoons finally arrived on the 25th, but too late, for many Confederates manned works on the hills west of Fredericksburg.
Burnside probed downriver for an unopposed crossing but, finding none, decided to cross at the town. On the morning of December 11th, Union engineers, hidden by fog, began laying the bridges. When the fog lifted, Confederate sharpshooters, hidden in buildings drove the engineers away. Union artillery responded with a massive barrage that reduced many buildings to rubble in Fredericksburg. The Confederate sharpshooters continued to pepper the engineers. Finally, Union volunteers, crossing in boats, dislodged the sharpshooters and secured the riverbank. On the 12th, the huge Union army crossed on 5 pontoon bridges. Burnside ordered a dawn assault against the 7-mile -long Confederate line.
Brig. Gen. William B. Franklin's Left Grand Division of 50,000 men launched the Union attack on the southern part of the field against Jackson's veterans, who were behind a railroad embankment. Two Union divisions penetrated a gap in the Confederate lines, pressing back Jackson's men. Confederate counterattacks sealed the breach, repulsing Franklin's assault. To the north, directly behind the town, Sumner's and Maj. gen. Joseph Hooker's Grand Divisions hurled themselves in suicidal attacks against Lee's impregnable position at Marye's Heights. Waves of Union soldiers stormed the stone walls, only to be engulfed in rifle and artillery fire. Piles of Union dead covered the field at nightfall.
Burnside, nearly overcome with grief, ordered a renewal of the attacks for the next day. His Grand Division commanders, however, convinced him of the futility of such an assault and Burnside finally revoked the order. Jackson argued for a decisive Confederate counterattack, but Lee demurred, for Union artillery across the river dominated the open terrain. The battle on the 13th cost the federals 12,653 in killed, wounded, and missing, while Confederate casualties amounted to only 5,309. During the night of the 15th, the Union army recrossed the river; Burnside had been decisively beaten.