They were fighting for control of the Mississippi.
Answer The Battle of Champion's Hill, just east of Vicksburg, was fought on 16 May 1863. This battle was fought out in the open and was a set-back for the Confederates. This forced them to return to the defenses of Vicksburg, which began the siege. The siege ended when General Pemberton surrendered Vicksburg on 3 July.
To hold the Mississipi river-port of Vicksburg, last Confederate stronghold on the river.
They surrounded the river-port of Vicksburg, the last big Confederate stronghold on the Mississippi.
In order to liberate the Mississippi. Vicksburg was the last major enemy garrison on the river.
Vicksburg was the last major Confederate stronghold on the Mississippi. Its capture by Grant ended the war in the West and divided the Confederacy into two parts.