On March 12, 1864 the Union War Department announced major changes in the Union command structure. General Henry Halleck, at his request, was named to a subordinate Union staff position, while Ulysses Grant assumed the general in chief of all armies post.
In addition, General Sherman was assigned as overall Union commander in the West, while General J.B. McPherson replaced Sherman as head of the Department of the Tennessee. Halleck had once been Grant’s superior officer and could not administratively remain over Grant, if Grant was to lead the Union army to victory.
And, William Tecumseh Sherman was a close friend and confidant to Ulysses Grant. Sherman had to be placed in a position where he could work for and in conjunction with Grant on the proposed Anaconda Plan.