On April 2-4, 1863, the Union army was camped on the north side of the Rappahannock River. Their General, Hooker, was moving towards General Lee, who was stationed across the river at Fredericksburg, Virginia. General Hooker tried catch Lee's army between two forces by splitting his army in two. He took up a position at Chancellorsville with part of his forces, and General Sedgwick took the remainder and crossed the river below Fredericksburg. As if he sensed what the Union army was doing, Lee launched a surprise attack and did the very same thing that Hooker was doing! His army surrounded Hooker's forces from all sides and Hooker's forces were completely defeated. The next day, he attacked and defeated General Sedgwick's forces.
During those eventful three days, the Confederate Army won a strategic battle.
At the Battle of Chancellorsville (May 1863), Robert E. Lee and General Thomas (Stonewall) Jackson, were able to defeat a numerically superior Union force, led by Gen. Joseph Hooker.
Because the Confederate forces were outnumbered, the conventional thing for them to do would have been to gather their forces in one place and try to avoid battle, or at the very least fight on the defensive.
Instead, Lee and Jackson decided to split their forces and use the tactic known as a flanking attack (attacking the enemy from the side rather than the front) against the Union army.
This tactic allowed the Confederates to win a victory that is still cited as a "stunning" win.
Their plan of attack was flanking, which was to attack from all sides.