Because Vicksburg was the last Confederate stronghold on the Mississippi.
By liberating the Mississippi, the Union would deny the Confederates all use of the great river, and isolate all Confederate units to the west of it.
You probably mean the battle of Vicksburg, in the American Civil War. According to http://www.answers.com/topic/battle-of-vicksburg, the capture of Vicksburg marked the turning point of the war.
The Battle of Vicksburg is also called the Siege of Vicksburg
Vicksburg was the true strategic and logistical turning point of the war.
Vicksburg was a Confederate stronghold on the Mississippi River
Vicksburg was to valuable as a river port to destroy.
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Vicksburg
Battle of Vicksburg
Vicksburg
The campaign to sieze Vicksburg took place over a four state area around Vicksburg, Mississippi (Tennessee, Arkansas, Louisiana, and Mississippi).
The purpose of capturing Vicksburg was to shut down a huge supply city the Confederacy relied on. Also, Vicksburg being on the Mississippi River, needed to fall allowing the Union full access to this key river. The problem was that Grant failed in attempts to capture Vicksburg early on in the war. His strategy of a river attack failed. Only later in the War did the siege of Vicksburg succeed.
Vicksburg