Battle of 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.
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.
When Hooker received the report about the Confederate movement, he thought that Lee might be starting a retreat, but he also realized that a flanking march might be in progress. He took two actions. First, he sent a message at 9:30 a.m. to the commander of the XI Corps, Maj. Gen. Oliver O. Howard on his right flank: "We have good reason to suppose the enemy is moving to our right. Please advance your pickets for purposes of observation as far as may be safe in order to obtain timely information of their approach."
At 10:50 a.m., Howard replied that he was "taking measures to resist an attack from the west." Hooker's second action was to send orders to Sedgwick – "attack the enemy in his front" at Fredericksburg if "an opportunity presents itself with a reasonable expectation of success" – and Sickles – "advance cautiously toward the road followed by the enemy, and harass the movement as much as possible". Sedgwick did not take action from the discretionary orders. Sickles, however, was enthusiastic when he received the order at noon. He sent Birney's division, flanked by two battalions of Col. Hiram Berdan's U.S. sharpshooters, south from Hazel Grove with orders to pierce the column and gain possession of the road.
But the action came too late. Jackson had ordered the 23rd Georgia Infantry to guard the rear of the column and they resisted the advance of Birney and Berdan at Catherine Furnace. The Georgians were driven south and made a stand at the same unfinished railroad bed used by Wright's Brigade the day before. They were overwhelmed by 5 p.m. and most were captured. Two brigades from A.P. Hill's division turned back from the flanking march and prevented any further damage to Jackson's column, which by now had left the area.
Most of Jackson's men were unaware of the small action at the rear of their column. As they marched north on Brock Road, Jackson was prepared to turn right on the Orange Plank Road, from which his men would attack the Union lines at around Wilderness Church. However, it became apparent that this direction would lead to essentially a frontal assault against Howard's line. Fitzhugh Lee met Jackson and they ascended a hill with a sweeping view of the Union position. Jackson was delighted to see that Howard's men were resting, unaware of the impending Confederate threat.
Although by now it was 3 p.m., Jackson decided to march his men two miles farther and turn right on the Turnpike instead, allowing him to strike the unprotected flank directly. The attack formation consisted of two lines—the divisions of Brig. Gens. Robert E. Rodes and Raleigh E. Colston—stretching almost a mile on either side of the turnpike, separated by 200 yards, followed by a partial line with the arriving division of A.P. Hill.

Significant contributions to the impending Union disaster were the nature of the Union XI Corps and the incompetent performance of its commander, Maj. Gen. Oliver O. Howard. Howard failed to make any provision for defending against a surprise attack, even though Hooker had ordered him to do so. The Union right flank was not anchored on any natural obstacle, and the only defenses against a flank attack consisted of two cannons pointing out into the Wilderness.
Also, the XI Corps was not well respected – an outfit with poor morale and no history of battlefield success. Many of its officers and enlisted men were immigrants from Germany and other parts of Central Europe, including a number of political refugees from the 1848 revolutions. The corps had been formed in the spring of 1862 by merging Brig. Gen Louis Blenker's division with Maj. Gen John C. Frémont's Mountain Department in West Virginia. After a miserable trek across Virginia in which Blenker's troops were provisioned inadequately and suffered from widespread hunger, disease, and desertion, they joined with Fremont in a campaign that resulted in them being soundly defeated by Stonewall Jackson.
Fremont's army became part of Maj. Gen John Pope's Army of Virginia in the summer. Fremont had refused to serve under Pope and was replaced by Maj. Gen Franz Sigel, an inept political general who, however, was much beloved by his German troops. Louis Blenker fell from a horse during the Northern Virginia campaign and suffered injuries that would claim his life later in 1863. The corps suffered heavy casualties at Second Bull Run and was left behind in Washington D.C. during the Maryland campaign. During the Fredericksburg campaign, it did not join the rest of the army until after the battle was over.
After Hooker took command, Sigel was the ranking general behind him. The XI Corps was the smallest in the army and Sigel's requests to general-in-chief Henry Halleck to have it enlarged were refused, so he resigned his command in March 1863 and was replaced by Maj. Gen Oliver O. Howard, who was widely unpopular with the enlisted men and brought in several new generals, such as Brig. Gen Francis Barlow, who had a reputation of being aggressive martinets. Eight of the 27 regiments in the corps had never been in battle before, while the remaining 21 had never been on the winning side of a battle. The German soldiers suffered from widespread ethnic friction with the rest of the army although a number of the regiments in the XI Corps consisted of native-born Americans.
Hooker had no major plans for the corps except for mopping up after the main battle was over, and it was placed out on the army's right flank where it was not expected to be involved in any fighting, and the woods to the west were assumed to be so thick that enemy troops could not possibly move through them and form a line of battle. As far as Hooker knew, the only possible route for a Confederate attack was along the turnpike, which would cause them to run right into the II and XII Corps, both elite outfits and well-entrenched. Further north, the Union line was held by the V Corps, also first-rate troops occupying an almost impregnable position.
As the day wore on, the men of the XI Corps became increasingly aware that something was going on in the woods to the west of them, but were unable to get any higher-ups to pay attention. Col. John C. Lee of the 55th Ohio received numerous reports of a Confederate presence out there, and Col. William Richardson of the 25th Ohio reported that huge numbers of Confederates were massing to the west. Col. Leopold von Gilsa, who commanded one of two brigades in Brig. Gen Charles Devens' division, went to Howard's headquarters warning him that an all-out enemy assault was imminent, but Howard insisted that it was impossible for the Confederates to get through the dense woods.
Maj. Gen Carl Schurz, who commanded the 3rd Division of the corps, began rearranging his troops into a line of battle. Captain Hubert Dilger, who commanded Battery I of the 1st Ohio Artillery, rode out on a reconnaissance mission, narrowly missed being captured by the Confederates, and rode far north, almost to the banks of the Rapidan, and back south to Hooker's headquarters, but a haughty cavalry officer dismissed his concerns and would not let him in to see the general. Dilger next went to Howard's headquarters, but was merely told that the Confederate army was retreating and that it was not acceptable to make scouting expeditions without permission of higher-ups. As the sun started to go down, all remained quiet on the XI Corps's front, the noises of the III and XII Corps engaging Lee's rear guard coming from off in the distance.

Around 5:30 p.m., Jackson turned to Robert Rodes and asked him "General, are you ready?" When Rodes nodded, Jackson replied, "You may go forward then."  Most of the men of the XI Corps were encamped and sitting down for supper and had their rifles unloaded and stacked. Their first clue to the impending onslaught was the observation of numerous animals, such as rabbits and foxes, fleeing in their direction out of the western woods.