Between 23 and 25 September 1863 the Eleventh and Twelfth Corps of the Army of the Potomac were sent across the Appalachians to strengthen Union troops in the struggle for supremacy in Eastern Tennessee. The Battle of Chickamaugaa Confederate victory that just missed being a complete Federal routhad ended, exhausting both armies and leaving Union troops perilously bottled up in Chattanooga. Upon the prodding of newspaperman Charles Dana, who was on the scene, Secretary of War Edwin Stanton resolved to provide Union commander General William Rosecrans with reinforcements sufficient to hold that key town. After a late-night meeting with Lincoln and other cabinet members, Stantons proposal to send more than twenty thousand men to Chattanooga by rail within a week was adopted.This is the story of that first great Federal troop movement by rail and of the corresponding Confederate transfer of General James Longstreets troops a few weeks previously. The contrast between the two operations points up the Union advantages in technology, infrastructure, and mobility that would offset the Confederate advantage of interior lines.An important study of a critical movement, the work also benefits from an assembly of anecdotes and comments from the troops involved.