"From that point on there was only a slim chance that independence could be won."

George, I know what you are talking about. But, splitting the Confederacy was a sideline benifit of controling the Mississippi river. The real purpose of controling the Mississippi River was to complete the Anaconda
Plan and isolate the eastern Confederacy from European trade goods.

As for the lost of Vicksburg reducing the chances of Independence for the South, well the southern military thinking, from the first, was to wear down the northern will to fight the war. It was the same thinking that the Japanese, the Viet Cong, the Taliban and Al-Queda have all thought that the Americans at home would grow tired of all the casualities and fighting and simply allow the southern States to leave.

The growing dissatisfaction in the Northern states with the war in 1864 was very much a part of Lincoln's re-election in November. McClellan, Lincoln's democrat opposition, ran on an antiwar platform