The Union army lay opposite Fredericksburg, looking at the fortified heights where they had received so bloody a repulse at the beginning of the winter. Hooker decided to distract the attention of the Confederates by letting a small portion of his force, under General Sedgwick, attack Fredericksburg, while he himself took the bulk of the army across the river to the right hand so as to crush Lee by an assault on his flank. All went well at the beginning, and on the first of May Hooker found himself at Chancellorsville, face-to-face with the bulk of Lee's forces; and Sedgwick, crossing the river and charging with the utmost determination, had driven out of Fredericksburg the Confederate division of Early; but when Hooker found himself in front of Lee he hesitated, faltered instead of pushing on, and allowed the consummate general to whom he was opposed to take the initiative.

Lee fully realized his danger, and saw that his only chance was, first to beat back Hooker, and then to turn and overwhelm Sedgwick, who was in his rear. He consulted with Jackson, and Jackson begged to be allowed to make one of his favorite flank attacks upon the Union army; attacks which could have been successfully delivered only by a skilled and resolute general, and by troops equally able to march and to fight. Lee consented, and Jackson at once made off. The country was thickly covered with a forest of rather small growth, for it was a wild region, in which there was still plenty of game. Shielded by the forest, Jackson marched his gray columns rapidly to the left along the narrow country roads until he was square on the flank of the Union right wing, which was held by the Eleventh Corps, under Howard. The Union scouts got track of the movement and reported it at headquarters, but the Union generals thought the Confederates were retreating; and when finally the scouts brought word to Howard that he was menaced by a flank attack he paid no heed to the information, and actually let his whole corps be surprised in broad daylight. Yet all the while the battle was going on elsewhere, and Berdan's sharpshooters had surrounded and captured a Georgia regiment, from which information was received showing definitely that Jackson was not retreating, and must be preparing to strike a heavy blow.