Chancellorsville

The Chancellorsville campaign pitted Union Army Maj. Gen. Joseph Hooker's Army of the Potomac against an army less than half its size, General Robert E. Lee's Confederate Army of Northern Virginia. Chancellorsville is known as Lee's "perfect battle" because his risky decision to divide his army in the presence of a much larger enemy force resulted in a significant Confederate victory. The victory, a product of Lee's audacity and Hooker's timid decision-making, was tempered by heavy casualties, including Lt. Gen. Thomas J. "Stonewall" Jackson. Jackson was hit by friendly fire, requiring his left arm to be amputated. He died of pneumonia eight days later, a loss that Lee likened to losing his right arm.
On May 1, Hooker advanced from Chancellorsville toward Lee, but the Confederate general split his army in the face of superior numbers, leaving a small force at Fredericksburg to deter Maj. Gen. John Sedgwick from advancing, while he attacked Hooker's advance with about four-fifths of his army. Despite the objections of his subordinates, Hooker withdrew his men to the defensive lines around Chancellorsville, ceding the initiative to Lee.

On May 2, Lee divided his army again, sending Stonewall Jackson's entire corps on a flanking march that routed the Union XI Corps. While performing a personal reconnaissance in advance of his line, Jackson was wounded by fire after dark from his own men close between the lines, and cavalry commander Maj. Gen. J. E. B. Stuart temporarily replaced him as corps commander.
On May 3 as Lee launched multiple attacks against the Union position at Chancellorsville, resulting in heavy losses on both sides and the pulling back of Hooker's main army. That same day, Sedgwick advanced across the Rappahannock River, defeated the small Confederate force at Marye's Heights in the Second Battle of Fredericksburg, and then moved to the west. The Confederates fought a successful delaying action at the Battle of Salem Church.

On May 4, Lee turned his back on Hooker and attacked Sedgwick, and drove him back to Banks' Ford, surrounding them on three sides. Sedgwick withdrew across the ford early on May 5. Lee turned back to confront Hooker who withdrew the remainder of his army across U.S. Ford the night of May 5–6.
Hooker, who began the campaign believing he had "80 chances in 100 to be successful", lost the battle through miscommunication, the incompetence of some of his leading generals (most notably Howard and Stoneman, but also Sedgwick), but mostly through the collapse of his own confidence. Hooker's errors included abandoning his offensive push on May 1 and ordering Sickles to give up Hazel Grove and pull back on May 2. He also erred in his disposition of forces; despite Abraham Lincoln's exhortation, "this time put in all your men," some 40,000 men of the Army of the Potomac scarcely fired a shot.