Siege: May 18-July 4, 1863
In May and June of 1863, Maj. Gen. Ulysses S. Grant's armies converged on Vicksburg, investing the city and entrapping a Confederate army under Lt. Gen. John Pemberton. On July 4, Vicksburg surrendered after prolonged siege operations. This was the culmination of one of the most brilliant military campaigns of the war. With the loss of Pemberton's army and this vital stronghold on the Mississippi, the Confederacy was effectively split in half. Grant's successes in the West boosted his reputation, leading ultimately to his appointment as General-in-Chief of the Union armies.
Source: American Battlefield Protection Program
To Ulysses S. Grant
July 13, 1863
I do not remember that you and I ever met personally. I write this now as a grateful acknowledgement for the almost inestimable service you have done the country. I wish to say a word further. When you first reached the vicinity of Vicksburg, I thought you should do, what you finally did - march the troops across the neck, run the batteries with the transports, and thus go below; and I never had any faith, except a general hope that you knew better than I, that the Yazoo Pass expedition, and the like, could succeed. When you got below, and took Port-Gibson, Grand Gulf, and vicinity, I thought you should go down the river and join Gen. Banks; and when you turned Northward East of the Big Black, I feared it was a mistake. I now wish to make the personal acknowledgment that you were right, and I was wrong.
Source: The Collected Works of Abraham Lincoln, Volume VI, pp. 327-368.