Military engagement of the American Civil War .
In May 1863 near Chancellorsville, Va., the Union army in Virginia, led by Joseph Hooker , attempted to encircle and destroy the Confederate Army of Northern Virginia, led by Robert E. Lee . The Union army was surprised by a flanking force under the command of Stonewall Jackson . Three days of fighting ended in a Union retreat north of the Rappahannock River. The Union army lost more than 17,000 men in a force of 130,000; the Confederate army lost more than 12,000, including Jackson, in a force of 60,000.