The Gathering Forces
General Robert E. Lee established his local headquarters in the city of Fredericksburg, Virginia by gathering forces from all across Virginia. By the beginning of the battle, he had amassed 60,892 men.1 This number would have increased by about 15,000 had Lieutenant General (Lt. Gen.) James Longstreet, near Norfolk, not been preoccupied with Federal forces near Suffolk, delaying his arrival until the battle was finished.*
Beginning on 27 April 1863, Major General (Maj. Gen.) Joseph Hooker marched four corps of the Army of the Potomac across the Rappahannock and Rapidan Rivers,2 mostly crossing near the junction of the two. A second force led by Maj. Gen. John Sedgwick crossed the Rappahannock near Fredericksburg,3 while a final force of cavalry under Maj. Gen. George Stoneman headed deep into Confederate territory to cut off their enemy's supply lines.
By the beginning of the fighting, the Union army had a force of 133,868,4 more than double that of the Confederate forces. Hooker had approximately 70,000 men in and around Chancellorsville;5 Sedgwick's forces numbered about 30,000,6 and Stoneman had about 7,500 cavalry.7
Stoneman's Raid: the Union Plan General Hooker planned to use a pincer movement to collapse upon the Confederates, four corps sneaking northwest first, then south to cross both rivers, and finally turning east to strike Lee's army in the rear (see attached image). The other two corps would hit Lee in the front, from Fredericksburg. All of this would be done while the cavalry destroyed Lee's supply lines.8 Hooker's confidence (and arguably arrogance) in his plan prompted him to say, "I have the finest army the sun every shone on. My plans are perfect, and when I start to carry them out, may God have mercy on General Lee, for I will have none!"9
The Confederate Plan General Lee thought it prudent to annihilate a portion of Hooker's forces before they could assemble against him. To this end, he planned to violate a long-standing military practice by dividing his forces while facing a superior enemy. One brigade (no more than about 4,000 men) would remain heavily fortified with Brigadier General (Brig. Gen.) William Barksdale on Marye's Heights. Maj. Gen. Jubal A. Early would defend Prospect Hill against any attack by Sedgwick with one division of 12,000 men. Meanwhile, Lt. Gen. Thomas J. "Stonewall" Jackson would lead 28,000 men of the Second Corps to attack the Union right flank and push through to Maj. Gen. Richard H. Anderson. A total of 40,000 men under Gen. Lee would confront 70,000 troops under Maj. Gen. Hooker at Chancellorsville.10
*More info can be found here.