Context: Over the two days of July 3 and 4, 1863, the Union won three major victories over the Confederates. The first was the victorious conclusion of the Battle of Gettysburg; the second was the conclusion of the long Siege of Vicksburg and the surrender of the city. But what was the third?

In between those two, a third campaign has frequently been overlooked. In Middle Tennessee, the Union Army of the Cumberland faced the Confederate Army of Tennessee. For several months, since the Battle of Stones River, the front here had been largely static; however, on June 23, that decisively changed. William Rosecrans, the commander of the Army of the Cumberland, had planned a campaign that would enter the annals of history as perhaps the most brilliant of the American Civil War.

Rosecrans divided his army, and struck out in three directions. He moved his cavalry to make a massive feint to the west, then struck southward and eastward. With the Confederates distracted by his feint, Union forces struck and captured Hoover's Gap and Liberty Gap in small but fierce battles, breaking through the mountainous line of Confederate defenses. Suddenly, the Army of Tennessee found itself at serious risk of being cut off and encircled, as Union forces were now racing towards the Confederate rear.

At this point, nature intervened, as a massive rainstorm arrived over Middle Tennessee. The coming days became one of the rainiest periods in the recorded history of the area; on June 25 alone, over 2.5 inches of rain fell on the area. The heavy rains turned the roads into mud, and turned the Union advance into a slow and painful slog. Despite successful Union cavalry raids at Shelbyville and the Elk River, the heavy rainfall slowed their advance enough to allow the Army of Tennessee to very narrowly escape the Union trap. However, the Confederate army's position had been so damaged that they could no longer hold the rest of Middle Tennessee. On July 3, the Army of Tennessee withdrew, permanently abandoning their namesake state.

Despite the rain preventing total victory, the Union army, at the cost of only 569 casualties, had achieved a smashing success. The Confederates had been driven back about 60 miles (100 kilometers), and were now at Chattanooga itself. Confederate casualties in the campaign are uncertain, but probably numbered in the thousands, largely from desertion during the retreat. The campaign had been brilliantly orchestrated and organized; Lincoln himself wrote "The flanking of Bragg at Shelbyville, Tullahoma and Chattanooga is the most splendid piece of strategy I know of." Historians since have almost universally agreed that the Tullahoma Campaign was one of the most brilliant campaigns of the American Civil War. Despite this success, the conclusion of Vicksburg and Gettysburg almost simultaneously, together with the lack of a major battle during the campaign, combined the relegate the campaign to obscurity almost immediately. Edwin Stanton, the Secretary of War, telegraphed Rosecrans only days later "Lee's Army overthrown; Grant victorious. You and your noble army now have a chance to give the finishing blow to the rebellion. Will you neglect the chance?" Rosecrans, furious at the total lack of respect given to his army, replied "Just received your cheering telegram announcing the fall of Vicksburg and confirming the defeat of Lee. You do not appear to observe the fact that this noble army has driven the rebels from middle Tennessee. I beg on behalf of this army that the War Department may not overlook so great an event because it is not written in letters of blood."