Thomas graduated from West Point in 1840, ranked 12 out of 42 in his class. On the day of graduation, he was assigned as second lieutenant in the 3rd U.S. Artillery. He served in the Regular Army for 20 years, during which time he rendered honorable and faithful service in the Seminole War from 1840-42; in command of various forts and barracks from 1842-45; in the military occupation of Texas in 1845-46; in the Mexican War, he fought at the battles of Fort Brown, Resaca de la Palma, Monterrey, and Buena Vista, receiving 3 promotions for distinguished gallantry in action; participating in nearly all its leading battles in the Seminole War in 1849-50.
From 1851-54, Thomas became an instructor of artillery and cavalry tactics at West Point. In 1855, he was appointed a major of the 2nd U.S. Cavalry by Jefferson Davis, then Secretary of War. He was on frontier duty at various posts in the interior of California and Texas, leading several expeditions against the Indians from 1855-60.
During these 20 years, he was repeatedly brevetted for gallant and meritorious services, rising through all the grades to a captain of artillery, and in 1855, he was made a major of the 2nd cavalry, which regiment he commanded for three years. He was wounded in a skirmish with the Indians at the headwaters of the Brazos river in August 1860. The following November, he went east on a leave of absence.
During the winter of 1860-61, Thomas watched with painful anxiety the culmination of that conflict of opinion which preceded the Civil War. He struggled with the decision of being loyalty to Virginia or loyal to the Union. He was expected to resign and join the Confederacy but decided to stay with the Union. Relinquishing his leave of absence, he reported for duty at Carlisle Barracks, Pennsylvania on April 14, 1861. This decision alienated him from his family and friends, and made him distrusted by the U.S. government. His family turned his picture against the wall, destroyed his letters, and never spoke to him again.
Thomas was promoted in rapid succession to be Lieutenant Colonel and Colonel in the Regular Army. At Major Gen. Robert Anderson's request, Col. William T. Sherman and Thomas were made Brigadier Generals of Volunteers and assigned to Anderson's command, the Department of the Cumberland. The first month's work that Thomas performed was at Camp Dick Robinson, Kentucky, where he mustered into service 11 regiments and 3 batteries of Ohio, Indiana, Kentucky and Tennessee troops, which he organized into the 1st Brigade.
On May 27, Thomas led a brigade from Chambersburg, Maryland to invade Virginia. A few days afterward, he led the right wing of Major Gen. Robert Patterson's army in the Battle of Falling Waters and defeated the Confederate forces under Brigadier Gen. Thomas J. "Stonewall" Jackson. In command of an independent force in eastern Kentucky, he attacked and routed the Confederates at the Battle of Mill Springs, gaining the first important Union victory in the war. The battle was by far the most important military success that had yet been achieved west of Virginia. It was the first victory in the department. In this battle, Thomas laid the foundation of his fame in the Army of the Center.
From November 30, 1861-September 30, 1862, Thomas commanded a division of Major Gen. Don C. Buell's army. Thomas was then appointed second in command of the Army of the Ohio, having previously refused the chief command. He served in that capacity in the Battle of Perryville and until October 30, when the old name of Department the Cumberland was restored and Major Gen. William S. Rosecrans assumed command. Thomas was assigned to the army's center command, which consisted of 5 divisions. He held this command in the Battle of Murfreesboro/Stone's River and until January 9, 1863, when he was assigned to command the XIV Corps during the Tullahoma Campaign and the Chickamauga Campaign. At the Battle of Chickamauga, he earned the name of "The Rock of Chickamauga," by some accounts being all that saved a terrible Union defeat from becoming a hopeless rout.
When Hood broke away from Atlanta in the fall, menaced Sherman's long line of communications, and endeavored to force Sherman to follow him, Sherman abandoned his communications and embarked on his "March to the Sea" Campaign. Thomas stayed behind to fight Hood's army. Thomas, with a smaller force, raced against Hood to reach Nashville, where he was to receive additional reinforcements.
At the Battle of Franklin, a large part of Thomas's force, under command of Major Gen. John M. Schofield, held Hood in check long enough to cover the concentration at Nashville. At Nashville, Thomas had to organize his forces, drawn from all parts of the West. He declined to attack until his army was ready and the ice covering the ground had melted enough for his men to move. The North, including Lieutenant Gen. Ulysses S. Grant himself, grew impatient at the delay. Major Gen. John A. Logan was sent with an order to supersede Thomas, and soon afterwards Grant left the Army of the Potomac to take command in person.
Before either arrived, Thomas made his attack against Hood and the Battle of Nashville was the most crushing defeat of any army on either side in the Civil War. For his brilliant victory, , one of the most decisive battles of the war, Thomas was made a major general in the Regular Army and received the "Thanks of Congress."
After the end of the Civil War, with most of his forces sent to other theaters of operations, Thomas remained in command in Tennessee until 1867. He then commanded the military department in Kentucky until 1869. In 1869, he was ordered to command the Division of the Pacific with headquarters at San Francisco.
President Andrew Johnson offered Thomas the rank of lieutenant general—with the intent to eventually replace Grant with him as General -in-Chief. Thomas asked the Senate to withdraw his name for that nomination because he didn't want to be party to politics.
Thomas died while in San Francisco of apoplexy, while writing an answer to an article criticizing his military career. He was buried in Oakwood Cemetery in Troy, New York.
Thomas was loved by his soldiers, for whom he always had a fatherly concern. He never had any ambitions outside of the military, remaining in the army his entire life. His victory at Nashville was a masterpiece, and for years it was the only Civil War battle studied in European military academies.
In the years following Thomas's death, Grant and Sherman, along with their partisans, attempted to minimize his importance and accomplishments. The veterans' organization for the Army of the Cumberland fought to see that he was honored for all he had done. Thomas' main legacy lay in his development of modern battlefield doctrine and in his mastery of logistics.
On September 27, 1864, after the capture of Atlanta, he was ordered by Sherman to return with a portion of his army into Tennessee and defend it against Major Gen. John B. Hood's invasion. Thus, Thomas was confronted by that veteran army which had so ably resisted Sherman on his march to Atlanta, and had to meet it with an effective force of about 40,000 infantry and 7,000 cavalry, having to remount the latter, provide transportation, and almost to organize and supply a new army. Although severely checked by Schofield at the Battle of Franklin, Hood gathered head and threatened Nashville.
Then the government and country waited impatiently for Thomas to attack, but be would not move until he was ready. Urgent despatches and orders rained in upon him, but he said they might remove him if they liked. An order removing him was actually made on December 9, but happily revoked.
On December 13, Logan was started for Nashville with orders to take the command on his arrival if Thomas had not moved, and 2 days later, Grant himself set out thither. On the road, both received the great news of the battle of December 15. Thomas had at length attacked, driving the Confederates 8 miles.
On the next day, Thomas completely redeemed his promise to "ruin Hood," whose army was broken to pieces and chased out of Tennessee. But even here, the victor was blamed as dilatory in the pursuit, although the reward of his splendid services could no longer be kept back. He received his commission as major-general in the Regular Army.
The nation was by this time ready to recognize Thomas' merits and to understand that it was solely by his remarkable abilities, without the influence of powerful friends, that he had attained a position second to that of no officer of the army. Honors and rewards were pressed upon him, but with a simple dignity of character he declined them all, satisfied with having done his duty.
Lieutenant Colonel - April 25, 1861
Colonel - May 3, 1861
Brigadier General USV- August 17, 1861
Major General USV- April 25, 1862
Brigadier General USA- October 27,1863
Major General USA- December 15, 1864
1st Brigade, 1st Division, Department of Pennsylvania (June-July 25,1861)
Department of the Shenandoah (July 25-August 17, 1861)
Banks' Division, Army of the Potomac (August 17-28, 1861)
Camp Dick Robinson, Kentucky, Department of the Ohio (October-December 2, 1861)
1st Division, Army of the Ohio (December 2, 1861 - April 30, 1862 and June 10 - September 29,1862)
Army of the Tennessee (April 30-June 10, 1862)
second in command of the Army of the Ohio (September 29-October 24, 1862)
Army of the Center, XIV Corps, Army of the Cumberland (November 5, 1862 - January 9, 1863)
IV Corps (January 9 - October 28,1863)
Army of the Center (October 28, 1863 - September 26, 1864)
Department of the Cumberland (October 28, 1863 June 27, 1865)