When the Civil War started the Union was hurt because they weren't an organized front as compared to the south. The Union had 3 Generals in the west compared to one in the south. Each Major General was in charge of his area or "Departments." The Union had a Department of Kansas, a Department of Missouri, and a Department of Ohio. All of these states were boarder states and very important for the Union to protect.
The Union made keeping Missouri and Kentucky an important objective. As a result of the Battle of Boonville, in June 1961, Missouri stayed in the Union and did not secede. Kentucky decided to remain neutral but eventually decided to stay in the Union (although Kentucky did supply troops for both sides during the war).
The Union forces continued to secure border states and in February 1962 won the Battle of Fort Henry in western Kentucky. From Kentucky Grant moved south along the Tennessee River into Conferate territory and took control of most of Tennessee. The most bloody battle at this point in the Civil War took place during this move down the Tennesee River. This bloody battle was the Battle of Shiloh.
As Grant was fighting Shiloh, Union troops (Maj. General Pope) were attacking "Island 10." This was another important battle since control of the water ways was very important. Island 10 was located the the Kentucky bend of the Mississippi River. The Union won at Island 10 and moved down the Mississippi. This lead to the fall of Memphis two months later. The Union continued to move south and eventually took control of New Orleans (the largest Confederate seaport).
At this point the Union had almost achieved control of the Mississippi River. Vicksburg was the last major area on the Mississippi that the Confederates controlled. Vicksburg was a long and very tough campaign. Eventually, after many attempts, Vicksburg fell to the Union. The Union now had complete control of the Mississippi.
One of the Union