Meanwhile, for the second time, Lee was dividing his army. Jackson would lead his Second Corps of 28,000 men around to attack the Union right flank while Lee exercised personal command of the remaining two divisions, about 13,000 men and 24 guns facing the 70,000 Union troops at Chancellorsville. For the plan to work, several things had to happen. First, Jackson had to make a 12-mile (19 km) march via roundabout roads to reach the Union right, and he had to do it undetected. Second, Hooker had to stay tamely on the defensive. Third, Early would have to keep Sedgwick bottled up at Fredericksburg, despite the four-to-one Union advantage there. And when Jackson launched his attack, he had to hope that the Union forces were unprepared.^[7]

Confederate cavalry under Stuart kept most Union forces from spotting Jackson on his long flank march, which started between 7 and 8 a.m. and lasted until midafternoon. Several Confederate soldiers saw the Union observation balloon Eagle soaring overhead and assumed that they could likewise be seen, but no such report was sent to headquarters. When men of the III Corps spotted a Confederate column moving through the woods, their division commander, Brig. Gen. David B. Birney, ordered his artillery to open fire, but this proved little more than harassment. The corps commander, Sickles, rode to Hazel Grove to see for himself and he reported after the battle that his men observed the Confederates passing for over three hours.^[8]