In November 1862, Union Gen. Ambrose E. Burnside led his 115,000-man army southward toward Richmond, the Confederate capital, but delays in receiving pontoon boats slowed his crossing of the Rappahannock River and gave Confederate Gen. Robert E. Lee time to block the advance near Fredericksburg with 78,000 men. On December 13, Burnside launched a two-pronged attack, the first across these fields, but although success seemed possible, Union troops suffered heavy casualties and retreated, while the second attack against the Confederate left on Marye's Heights fared even worse, with no Federal soldiers reaching the Confederate line. Two days later, Burnside withdrew across the Rappahannock after Lee inflicted 13,000 casualties on the Union army while losing 5,000 men. Lee followed this victory with another at Chancellorsville that spring and soon began an invasion of the North.