Siege of Vicksburg

"The Confederate surrender on July 4, 1863, is sometimes considered, when combined with Gen. Robert E. Lee's defeat at Gettysburg by Maj. Gen. George Meade the previous day, the turning point of the war. It cut off the Trans-Mississippi Department (containing the states of Arkansas, Texas and part of Louisiana) from the rest of the Confederate States, effectively splitting the Confederacy in two for the rest of the war. Lincoln called Vicksburg "the key to the war."[6]" - https://en.wikipedia.org/wiki/Siege_of_Vicksburg