Gettysburg.

I don't think a Confederacy at Gettysburg would change the outcome of the war. They didn't have a shot at getting Britain or France as their allies. If they actually did, it would have caused much controversy and people would start accusing the foreign Britain or France for being hypocritical of supporting slavery when they had already outlawed it a long time ago. They probably did not want that to happen. Both sides suffered major losses at Gettysburg, but the Confederacy suffered a lot more since Lee lost many of his commanding generals as well as men he couldn't replace who died/ were injured in the battle. Lincoln probably still would have still had some hope since at the time the Union won Gettysburg, there was also the surrender of Vicksburg, Mississippi, which was also a major victory which gave the Union complete control of the Mississippi River. Even if the Union did lose the Battle of Gettysburg, they still would have the better chance of winning the war.