On November 14, Burnside, now in command of the Army of the Potomac, sent a corps to occupy the vicinity of Falmouth near Fredericksburg. The rest of the army soon followed. Lee reacted by entrenching his army on the heights behind the town. On December 11, Union engineers laid five pontoon bridges across the Rappahannock under fire. On the 12th, the Federal army crossed over, and on December 13, Burnside mounted a series of futile frontal assaults on Prospect Hill and Marye's Heights that resulted in staggering casualties. Meade's division, on the Union left flank, briefly penetrated Jackson's line but was driven back by a counterattack. Union generals C. Feger Jackson and George Bayard, and Confederate generals Thomas R.R. Cobb and Maxey Gregg were killed. On December 15, Burnside called off the offensive and recrossed the river, ending the campaign. Burnside initiated a new offensive in January 1863, which quickly bogged down in the winter mud. The abortive "Mud March" and other failures led to Burnside's replacement by Maj. Gen. Joseph Hooker in January 1863.