The Union Army took a series of military actions during the Civil War in which their troops took control of the Mississippi River. Mississippi was the main north-south avenue of transport. In July 1863, the western part of the Confederate states was split from the states east of the Mississippi River. Plan of the Union Army generals was to defeat the Confederacy through blockade of ports and control of rivers. Answer: Union forces took control of Mississippi River because they wanted to cut off the eastern part of Confederacy and to control the supply lines.
Union forces wanted control not only of the Mississippi River, but the Ohio, Tennessee, and Cumberland Rivers (which they achieved by capturing Ft. Henry and Ft. Donelson). The Mississippi gave them a water route to use for movement of troops and supplies into the South, and by following the River to New Orleans and capturing the city, they were able to close off a major port of the Confederacy. (Control of the other rivers also provided a route to penetrate farther into the south with the assurance of the river to facilitate supply lines -- without that necessity, they were significantly limited in their ability to advance, needing proximity to Union-controlled territory or a railroad, and railroads in the South were few.)
