On July 4, 1863 the Confederates in Vicksburg surrendered to General Grant after a siege that lasted about six weeks. What was so significant
about this surrender?
The Confederates lost the Civil War.
The Confederates now controlled the Mississippi River.
The Union now had complete control of the Mississippi River.
The Union won the Civil War.

The Union now had complete control of the Mississippi River.

Explanation:

The Mississippi river divided the Confederate states in two and therefore was a major aim of the Union during the Civil War because they knew that if they could capture it, they could then proceed to defeat the two Confederate parts with more ease.

Generals Grant and Sherman fought many battles around the area in order to capture Vicksburg which was on the the river and after a long siege eventually did so. In capturing the city and gaining control of the Mississippi, Union morale was renewed and this contributed in no small part to Union efforts to end the war.