What caused the Union's momentum against the South in the Western Theater to slow down?

Answer 1

Major General Henry W. Halleck began his successful conquest in the Western Theater. He secured nominal control over most of Missouri, and took over large parts of the Confederate state of Tennessee. Under his leadership Union forces also penetrated into northern Mississippi and Alabama. At that point his advances began to slow down. The main reason being that the secure supply and operational communications provided by the Tennessee River ended. At that point the Union's problems with logistics emerged. The vast size of the South, its lack of good roads and the limited opportunities of foraging for supplies became evident. At that point, based on his successes to date, he was appointed by President Lincoln to become the general in chief and Halleck's attention had to be focused on the entirety of the Union's war efforts.