The Battle of Malvern Hill

This battle was fought on July 1, 1862. Few engagements ever fought had a more positive ultimate result from a negative outcome than did the Battle of Malvern Hill. This was the culminating engagement of the Seven Days Battles, during which Gen. Joe Johnson was wounded and General Robert E. Lee took command of the Confederate army. The battle was badly managed on the Southern side and resulted in a resounding Yankee victory. 

Lee was deeply disappointed in the outcome, and several commanders were reassigned. More importantly, the Army was reorganized into two Corps, commanded by Generals Stonewall Jackson and James Longstreet, with Gen. Jeb Stuart commanding the cavalry. From that time until the Battle of Chancellorsville in May 1863, the Army of Northern Virginia was one of the most formidable fighting forces in the history of warfare. 

TROOP STRENGTHS ACCORDING TO WIKIPEDIA:

Second Bull Run (South was outnumbered 77M to 50M)

Antietam (outnumbered 87M to 38M)

Fredericksburg (outnumbered 122M to 78M)

Chancellorsville (outnumbered 133M to 60M)

These battles resulted in smashing victories for the South despite the overwhelming numerical odds, with the exception of Antietam, which was a tactical draw, although a strategic victory for the North, since it stopped the Southern movement into Maryland. Before the Battle of Antietam, of course, the North had accidentally come into possession of the Southern battle plans, and knew that Lee had divided his army and sent Gen. A. P. Hill to Harper's Ferry. 

At Chancellorsville, General Jackson was wounded (and later died), and Lee was forced once again to reorganize his army. It had some remarkable accomplishments after that time, but was never again quite the devastating force it had been while this quartet of generals were in harness together - which organization resulted from an embarrassing loss at Malvern Hill.