What took place in 1863 to swing the advantage to the side of the Union?

The Union won three decisive battles, which swung the advantage to its side:

1 - The Battle and Siege of Vicksburg, which, after a campaign lasted from May19 through July 4, 1863, led to the surrendering of the Confederate fortress and the capture of the besieged Pemberton's Army, followed on July 9 by the surrendering of Port Hudson. The Mississippi River was then under complete control of the Union.

2 - The Battle of Gettysburg, fought from July 1 to July 3, 1863.

3- The Third Battle of Chattanooga, fought from October 26 through November 25 1863, which opened the way to the invasion of Georgia.