The South won every major Civil War battle until

The South won major early battles such as First Bull Run, Second Bull Run, and Chancellorsville. At Antietam in September 1862, Union forces stopped Lee’s invasion of the North, making it a strategic Union victory even though the battle itself was costly and inconclusive. So the battle that ended the South’s string of major victories was the Battle of Antietam.

AI-written and checked against the verified answer.