Short answers, pulled from the story.
The western theater of the American Civil War encompassed major military operations in Alabama, Georgia, Florida, Mississippi, North Carolina, Kentucky, South Carolina and Tennessee. It also included Louisiana east of the Mississippi River.
Union operations began in September 1861 with attempts to secure Kentucky. More than half of the state was under Confederate control by late 1861 into 1862.
General Albert Sidney Johnston commanded the Confederate force at the Battle of Shiloh. The Army of the Tennessee and the Army of the Ohio defeated this Confederate Army of Mississippi.
Abraham Lincoln believed that the river fortress city of Vicksburg, Mississippi, was a key to winning the war because it prevented full Union control of the Mississippi River. Situated on high bluffs overlooking a sharp bend in the river, Vicksburg was called the Gibraltar of the Mississippi.
Sherman avoided frontal assaults against most of Johnston's positions instead maneuvering in flanking marches around the Confederate defenses. This approach allowed him to drive Johnston southeast through mountainous terrain starting in May and June 1864.