Answer:
The Union won the battle of Gettysburg, which tipped the scales away from the Confederates (who up until this point were winning). Some important Confederate victories prior to Gettysburg include The First Battle of Bull Run, Seven Days Battle, and the Battle of Fredericksburg.
Explanation:
Because of the Confederate defeat at Gettysburg, one of the best Confederate armies were defeated, that of Robert E. Lee. The Confederate defeat at Gettysburg also forced them back into the South, and they would never again fight in the North. The confederate moral was also significantly harmed by the defeat at Gettysburg, for example, General Lee no longer had the confidence to invade the North.
