Battle of Chancellorsville
Catharine Furnace, abandoned in the 1840s, had recently been reactivated to produce iron for the Confederate war effort. This area was largely unsuitable for the deployment of artillery and the control of large infantry formations, which would nullify some of the Union advantage in military power. It was important for Hooker's plan that his men move quickly out of this area and attack Lee in the open ground to the east. There were three primary roads available for this west-to-east movement: the Orange Plank Road, the Orange Turnpike, and the River Road.
The Confederate dispositions were as follows: the Rappahannock line at Fredericksburg was occupied by Longstreet's First Corps division of Lafayette McLaws on Marye's Heights, with Jackson's entire Second Corps to their right. Early's division was at Prospect Hill and the divisions of Rodes, Hill, and Colston extended the Confederate right flank along the river almost to Skinker's Neck. The other division present from Longstreet's Corps, Anderson's, guarded the river crossings on the left flank. Stuart's cavalry was largely in Culpeper County near Kelly's Ford, beyond the infantry's left flank.

Initial movements: 

April 27–30: Movement to battle: On April 27–28, the initial three corps of the Army of the Potomac began their march under the leadership of Slocum. They crossed the Rappahannock and Rapidan rivers as planned and began to concentrate on April 30 around the hamlet of Chancellorsville, which was little more than a single large, brick mansion at the junction of the Orange Turnpike and Orange Plank Road. Built in the early 19th century, it had been used as an inn on the turnpike for many years, but now served as a home for the Frances Chancellor family. Some of the family remained in the house during the battle.
Hooker arrived late in the afternoon on April 30 and made the mansion his headquarters. Stoneman's cavalry began on April 30 its second attempt to reach Lee's rear areas. Two divisions of II Corps crossed at U.S. Ford on April 30 without opposition. By dawn on April 29, pontoon bridges spanned the Rappahannock south of Fredericksburg and Sedgwick's force began to cross.
Pleased with the success of the operation so far, and realizing that the Confederates were not vigorously opposing the river crossings, Hooker ordered Sickles to begin the movement of the III Corps from Falmouth the night of April 30 – May 1. By May 1, Hooker had approximately 70,000 men concentrated in and around Chancellorsville.

In his Fredericksburg headquarters, Lee was initially in the dark about the Union intentions and he suspected that the main column under Slocum was heading towards Gordonsville. Jeb Stuart's cavalry was cut off at first by Stoneman's departure on April 30, but they were soon able to move freely around the army's flanks on their reconnaissance missions after almost all their Union counterparts had left the area.
As Stuart's intelligence information about the Union river crossings began to arrive, Lee did not react as Hooker had anticipated. He decided to violate one of the generally accepted principles of war and divide his force in the face of a superior enemy, hoping that aggressive action would allow him to attack and defeat a portion of Hooker's army before it could be fully concentrated against him. He became convinced that Sedgwick's force would demonstrate against him, but not become a serious threat, so he ordered about 4/5 of his army to meet the challenge from Chancellorsville. He left behind a brigade under Brig. Gen. William Barksdale on heavily fortified Marye's Heights behind Fredericksburg and one division under Maj. Gen. Jubal A. Early, on Prospect Hill south of the town.
These roughly 11,000 men and 56 guns would attempt to resist any advance by Sedgwick's 40,000. He ordered Stonewall Jackson to march west and link up with Maj. Gen. Richard H. Anderson's division, which had pulled back from the river crossings they were guarding and began digging earthworks on a north–south line between the Zoan and Tabernacle churches. McLaws's division was ordered from Fredericksburg to join Anderson. This would amass 40,000 men to confront Hooker's movement east from Chancellorsville. Heavy fog along the Rappahannock masked some of these westward movements and Sedgwick chose to wait until he could determine the enemy's intentions.

Opposing forces: 

Union: The Army of the Potomac, commanded by Maj. Gen. Joseph Hooker, had 133,868 men and 413 guns organized as follows:

I Corps, commanded by Maj. Gen. John F. Reynolds, with the divisions of Brig. Gens. James S. Wadsworth, John C. Robinson, and Maj. Gen. Abner Doubleday.
II Corps, commanded by Maj. Gen. Darius N. Couch, with the divisions of Maj. Gens. Winfield Scott Hancock, William H. French, and Brig. Gen. John Gibbon.
III Corps, commanded by Maj. Gen. Daniel E. Sickles, with the divisions of Brig. Gen. David B. Birney, and Maj. Gens. Hiram G. Berry and Amiel W. Whipple.
V Corps, commanded by Maj. Gen. George G. Meade, with the divisions of Brig. Gens. Charles Griffin, Andrew A. Humphreys, and Maj. Gen. George Sykes.
VI Corps, commanded by Maj. Gen. John Sedgwick, with the divisions of Brig. Gens. William T. H. Brooks, Albion P. Howe, Maj. Gen. John Newton, and Col. Hiram Burnham.
XI Corps, commanded by Maj. Gen. Oliver O. Howard, with the divisions of Brig. Gens. Charles Devens, Jr., Adolph von Steinwehr, and Maj. Gen. Carl Schurz.
XII Corps, commanded by Maj. Gen. Henry W. Slocum, with the divisions of Brig. Gens. Alpheus S. Williams and John W. Geary.
Cavalry Corps, commanded by Maj. Gen. George Stoneman, with the divisions of Brig. Gens. Alfred Pleasonton, William W. Averell, and David M. Gregg.

Confederate: Gen. Robert E. Lee's Army of Northern Virginia fielded 60,298 men and 220 guns, organized as follows:

First Corps, commanded by Lt. Gen. James Longstreet. Longstreet and the majority of his corps (the divisions of Maj. Gen. John Bell Hood and Maj. Gen.  George E. Pickett, and two artillery battalions) were detached for duty in southeastern Virginia. The divisions present at Chancellorsville were those of Maj. Gens. Lafayette McLaws and Richard H. Anderson.
Second Corps, commanded by Lt. Gen. Stonewall Jackson, with the divisions of Maj. Gen. A.P. Hill, Brig. Gen. Robert E. Rodes, Maj. Gen. Jubal A. Early, and Brig. Gen. Raleigh E. Colston.
Cavalry Corps, commanded by Maj. Gen. J.E.B. Stuart. (Stuart's corps had only two brigades at Chancellorsville, those of Brig. Gens. Fitzhugh Lee and W.H.F. "Rooney" Lee. The brigades of Brig. Gens. Wade Hampton and William E. "Grumble" Jones were detached.)
The Chancellorsville campaign was one of the most lopsided clashes of the war, with the Union's effective fighting force more than twice the Confederates', the greatest imbalance during the war in Virginia. Hooker's army was much better supplied and was well-rested after several months of inactivity. Lee's forces, on the other hand, were poorly provisioned and were scattered all over the state of Virginia. Some 15,000 men of Longstreet's Corps had previously been detached and stationed near Norfolk in order to block a potential threat to Richmond from Federal troops stationed at Fort Monroe and Newport News on the Peninsula, as well as at Norfolk and Suffolk.
In light of the continued Federal inactivity, by late March Longstreet's primary assignment became that of requisitioning provisions for Lee's forces from the farmers and planters of North Carolina and Virginia. As a result of this the two divisions of Maj. Gen. John Bell Hood and Maj. Gen.  George Pickett were 130 miles (210 km) away from Lee's army and would take a week or more of marching to reach it in an emergency. After nearly a year of campaigning, allowing these troops to slip away from his immediate control was Lee's gravest miscalculation. Although he hoped to be able to call on them, these men would not arrive in time to aid his outnumbered forces.