This Virginia-born general, noted as the "Rock of Chickamauga" for his determined stand on Snodgrass Hill, his defensive position in Chattanooga and his destruction of General John Bell Hood's army in Nashville, was once considered by Abraham Lincoln to command the Union forces. Lincoln rejected him because he was from Virginia.

First active duty for Thomas was in the Shenandoah Valley in his home state of Virginia. He was quickly promoted to general and won decisive victories throughout the Western Theater. In January, 1863, he becomes commander of the 14th Corps, Army of the Cumberland, reporting to William Rosecrans. Over the next 9 months his corps advanced with the rest of Rosecrans army, rarely engaging in significant combat, a testament to the uncanny strategic mind of Rosecrans. It was near Chickamauga Creek, on September 20, 1863, however, that Thomas gained his greatest fame. After General John B. Hood exploited a hole in the Union line and forced Rosecrans from the field of battle, Thomas organized a weak line of men along a modest ridge near the Snodgrass cabin. At times giving orders directly to enlisted men, Thomas, backed by the timely arrival of Gordon Granger and his reserves, rallied to halt the Rebel advance. At evening he began a staggered withdrawal to Chattanooga.

During the Atlanta Campaign Thomas repeatedly defeated the Army of Tennessee commanders Joe Johnston and John Bell Hood. His only major mistake came when he ordered a late afternoon attack on what he felt was the end of the Dallas Line, near Pickett's Mill in present-day Paulding County.

Thomas withstood the brunt of Hood's First Sortie (battle of Peachtree Creek). During the March to the Sea Thomas moved to Tennessee to protect against John Bell Hood, which he did successfully at the battles of Franklin and Nashville.