Things were not going much better for the Federals in the east. There were high hopes after Lee's Army of Northern Virginia had been turned back at Antietam. Some speculated that another battle might crush him. But as usual, McClellan was reluctant to be aggressive; he kept his army north of the Potomac for weeks while Lee rested and refitted his army back in Virginia. President Lincoln finally grew impatient and replaced McClellan with Burnside.

Burnside wanted to take the Army of the Potomac directly south the 100 miles between Washington, D.C., and Richmond, Virginia. Directly on that line was the town of Fredericksburg on the southern side of the Rappahan-nock River, and Burnside marched the army to the north side in good order. If he had been able to cross the river then, he could have taken the town and moved on, but the pontoon bridges he needed were misplaced. While Burnside waited, Lee moved his Army of Northern Virginia to the heights behind the town.

Burnside finally got his bridges and crossed the river in mid-December. He set out his army in full panoply and advanced on December 13, but the Confederates had built their defenses well. Although Burnside launched charge after charge and the Union men showed themselves to be as brave as any soldiers could be, they had no chance of success against the well-placed Confederate defenders. The slaughter was especially astonishing on a sloping field in front of a stone wall behind the city in an area called Marye's Heights. Burnside had to admit defeat and withdrew to the north side of the river again. Union killed, wounded, and missing totaled 12,653, while the Confederate total amounted to less than half as many — 5,309. The battle ruined Burnside's effort at overall command and showed the folly of attacking uphill at well-established Confederate entrenchments.