A native of Virginia, George H. Thomas graduated from West Point in 1840 with William T. Sherman and R.S. Ewell. Commissioned into the artillery, he participated in punitive campaigns against Native American tribes in Florida and on the Plains. His conduct during the Mexican War earned him brevets for gallantry at Monterrey and Buena Vista.
Despite being a Southern-born, Thomas remained loyal to the Union. During the opening weeks of the war, he commanded a brigade in the Shenandoah Valley, after which he was sent to Kentucky to organize new recruits. In January 1862, Thomas served as second in command of the 1st Division Army of the Ohio, helping to secure a victory at Mill Springs. Later that spring, he commanded a division at Shiloh and in the advance on Corinth, Mississippi.
Later that year Thomas returned to Kentucky, where he turned down an offer to replace Don Carlos Buell as Army of the Ohio commander. When Rosecrans accepted the position, Thomas became commander of the XIV Corps, Army of the Cumberland, demonstrating his capability at Stone's River and during the 1863 Tullahoma Campaign.
Affectionately referred to by his troops as "Pap," his stout defense at Chickamauga in September 1863 earned him sobriquet of the "Rock of Chickamauga," making him a national hero. His steady resolve during that battle allowed Rosecrans and his troops to retreat safely.
Having proved a reliable and capable leader, Thomas was promoted to command the Army of the Cumberland. On November 25, 1863, his troops launched an aggressive assault on Missionary Ridge that ended in a decisive Confederate defeat. He checked the advancing Army of Tennessee, led by John B. Hood, at Franklin, Tennessee, on November 30, 1864, after which retreated to Nashville.
Despite Grant's repeated orders to launch an immediate offensive against Hood's forces, Thomas delayed, fearing that his army was too weak. Grant ordered a replacement to relieve Thomas of command, but Thomas's army rallied before the replacement arrived. On December 15, 1864, Thomas launched a crushing attack against the Confederates. Under his command, the Union army routed and nearly destroyed Hood's army, effectively ending Hood's career in the field. The Battle of Nashville was an overwhelming victory for the Union and one of the decisive battles of the war. Thomas was promoted to major general in the regular army and awarded the Thanks of Congress for his leadership.