The city of Vicksburg is located on the Mississippi River. It was the last major port on the river held by the South. If the North could take Vicksburg, the Confederacy would be cut off from supply lines to the west. Also, rebel states such as Texas, Louisiana, and Arkansas would be isolated from the rest of the South.
The Siege of Vicksburg was the end of a long series of battles in the western theatre of the Civil War called the Vicksburg Campaign. The Union Army, led by General Grant, had won a number of battles against the Confederates pushing them back towards Vicksburg. They also captured the city of Jackson, the capital of Mississippi. Grant approached the city slowly, forcing the Confederates to retreat before him. While approaching the city, he captured the local railroad and secured his own supply lines while isolating the city of Vicksburg.
Grant then decided to lay siege to the city. He would bomb them constantly and wait until they ran out of food. He knew that eventually they would have to surrender. The conditions in the city got worse and worse over the next several weeks. The people in the city began to run out of food. They started to eat anything available including the horses, dogs, and cats. Near the end they were even eating rats and tree bark. Because of malnutrition, many of the soldiers became sick from diseases like scurvy, dysentery, and malaria.