The Battle of Gettysburg is described by some historians as the South’s last chance to win the war. When they lost this battle, they lost that chance.
The Battle of Gettysburg occurred during a Southern invasion of the North. Lee had invaded the North in the hope that the invasion would encourage Northerners who wanted to end the war. He also hoped that some foreign country like Britain or France would at least recognize the Confederacy, making it harder for the Union to win.
When the South lost at Gettysburg, it was forced to retreat and the hopes Southerners had for the invasion were dashed.