George Henry Thomas in the context of "Battle of Chickamauga"

The American Civil War significantly affected Tennessee, with every county witnessing combat. During the War, Tennessee was a Confederate state, and the last state to officially secede from the Union to join the Confederacy. Tennessee had been threatening to secede since before the Confederacy was even formed, but would not officially do so until after the fall of Fort Sumter when public opinion throughout the state drastically shifted. Tennessee seceded in protest to President Lincoln's April 15 Proclamation calling forth 75,000 members of state militias to suppress the rebellion. Tennessee provided the second largest number of troops for the Confederacy, and would also provide more southern unionist soldiers for the Union Army than any other state within the Confederacy.

In February 1862, some of the war's first serious fighting took place along the Tennessee and Cumberland rivers, recognized as major military highways, and mountain passes such as Cumberland Gap were keenly competed-for by both sides. The Battle of Shiloh and the fighting along the Mississippi brought glory to the then little-known Ulysses S. Grant, while his area commander Henry Halleck was rewarded with a promotion to General-in-Chief. The Tullahoma campaign, led by William Rosecrans, drove the Confederates from Middle Tennessee so quickly that they did not take many casualties, and were strong enough to defeat Rosecrans soon afterward. At Nashville in December 1864, George Thomas routed the Army of Tennessee under John Bell Hood, the last major battle fought in the state.

The Chattanooga campaign was a series of maneuvers and battles in October and November 1863, during the American Civil War. Following the defeat of Maj. Gen. William S. Rosecrans's Union Army of the Cumberland at the Battle of Chickamauga in September, the Confederate Army of Tennessee under Gen. Braxton Bragg besieged Rosecrans and his men by occupying key high terrain around Chattanooga, Tennessee. Maj. Gen. Ulysses S. Grant was given command of Union forces in the West which was now consolidated under the Division of the Mississippi. Significant reinforcements also began to arrive with him in Chattanooga from Mississippi and the Eastern Theater. On October 18, Grant removed Rosecrans from command of the Army of the Cumberland and replaced him with Major General George Henry Thomas.

During the opening of a supply line (the "Cracker Line") to feed the starving men and animals in Chattanooga, a force under Maj. Gen. Joseph Hooker fought off a Confederate counterattack at the Battle of Wauhatchie on October 28–29, 1863. On November 23, the Army of the Cumberland advanced from the fortifications around Chattanooga to seize the strategic high ground at Orchard Knob while elements of the Union Army of the Tennessee under Maj. Gen. William Tecumseh Sherman maneuvered to launch a surprise attack against Bragg's right flank on Missionary Ridge. On November 24, Sherman's men crossed the Tennessee River in the morning and then advanced to occupy high ground at the northern end of Missionary Ridge in the afternoon. The same day, a mixed force of almost three divisions under Maj. Gen. Joseph Hooker defeated the Confederates in the Battle of Lookout Mountain. The next day they began a movement toward Bragg's left flank at Rossville.