American Civil War: The Battle of Gettysburg begins when General Robert E. Lee attempts to invade the North. Fought in and around the town of Gettysburg, Pennsylvania, this will generally be considered the biggest turning point of the Civil War.
It's the most aggressive move made by the South — and the South loses. If he had won, he might have undermined morale in the North and encouraged Lincoln to seek peace. Instead, Lee doesn't make any more aggressive moves and just reacts to aggression from the North.