(May 1–5, 1863), in the American Civil War, bloody assault by the Union army in Virginia that failed to encircle and destroy the Confederate Army of Northern Virginia. Following the “horror of Fredericksburg” (December 13, 1862), the Confederate army of General Robert E. Lee (Lee, Robert E.) and the Union force under General Joseph Hooker (Hooker, Joseph) had spent the winter facing each other across the Rappahannock River in Virginia. On April 27 Hooker dispatched his cavalry behind Lee's army, intending to cut off a retreat. Two days later he sent a small diversionary force across the Rappahannock below Fredericksburg and crossed upriver with the main body of his army. By May 1 his superior forces were massed near Chancellorsville, a crossroads in a densely forested lowland called the Wilderness. Deprived of his cavalry, however, Hooker was blind to Lee's movements, and on May 2, when Lee ordered General Thomas J. “Stonewall” Jackson (Jackson, Thomas Jonathan)'s “foot cavalry” to swing around and attack the Union right, Hooker's surprised flank was routed. The Union general withdrew, and Lee's pressure over the next three days forced a Union retreat north of the river. The South's greatest casualty was the loss of Jackson, who died May 10 of battle wounds. Of 130,000 Union soldiers engaged at Chancellorsville, more than 17,000 were lost; of 60,000 Confederates, more than 12,000 were lost.