Washington sent Joseph Hooker with roughly 20,000 men, ordered William T. Sherman to move another corps, and then placed Ulysses S. Grant over the new Military Division of the Mississippi. That command joined territory from the Appalachians to the Mississippi under one officer for the first time. Grant chose George Thomas to replace Rosecrans because he expected Thomas to carry out his direction, while Thomas promised to hold Chattanooga even at the risk of starvation. Union transport capacity and centralized command were part of the same response because reinforcements needed an authority able to coordinate them.
Chapter · 24:26-29:32
Grant unified the Union response to Chattanooga
The Union concentrated authority and reinforcements under Grant, who replaced Rosecrans with Thomas and coordinated armies across the western theater.