Today marks one of the turning points in American history. Vicksburg, Mississippi succumbed to a lengthy Northern siege on July 4, 1863, the day after the defeat of the Army of Northern Virginia at Gettysburg. With the fall of Vicksburg, the Confederate States were cut in two, and the Union Army controlled the Mississippi River.
From the Wikipedia entry:
The Confederate surrender following the siege at Vicksburg is sometimes considered, when combined with Gen. Robert E. Lee's defeat at Gettysburg the previous day, the turning point of the war. It also cut off communication with Confederate forces in the Trans-Mississippi Department for the remainder of the war. The city of Vicksburg would not celebrate Independence Day for about eighty years as a result of the siege and surrender.