United States Army Officer. He served during the Indian Wars as Captain and commander of Company D, 7th United States Regular Cavalry, which he led during the Battle of the Little Big Horn. Assigned to Major Marcus Reno's Battalion during the battle, he survived the fight, but he is reported to have drunk himself to death from his remorse over losing his commanding officer, General George A. Custer, whom he thought highly of. During the Battle, he disobeyed orders and led an effort to go help Custer that was driven back by a larger force of Indians. Born in Nashville, Ohio, to William and Martha (Stewart) Weir. When Thomas was a young child, his family moved to Albion, Michigan, where he attended school and later graduated from the University of Michigan in June 1861. He enlisted August 27, 1861 in Company B, 3rd Michigan Volunteer Cavalry, and was rapidly promoted to First Sergeant. Promoted to 2nd Lieutenant on October 13, 1861, he fought at New Madrid, siege of Corinth, Farmington, Iuka, Coffeeville, and the second battle of Corinth. Appointed First Lieutenant on June 19, 1862, he was taken prisoner by Confederates on June 26, 1862. Released on January 8, 1863, he rejoined his unit, but had been appointed Captain, November 1, 1862, while still a prisoner. He then served as Assistant Inspector General on the staff of Major General George Custer (his future commanding officer in the 7th Cavalry after the war). Brevetted Major, US Volunteers on January 18, 1865 and to Lieutenant Colonel, US Volunteers on July 31, 1867, in the realignment of the United States Army after the Civil War, he was appointed Captain, 7th US Cavalry, on July 31, 1867. During the Battle of the Little Big Horn, he was assigned to Major Reno's Battalion. After Reno's Battalion retreated to the hilltop position, he attempted to go to Custer's rescue with his company, but was driven back by the Indians. His closest approach to Custer's final position is called Weir Point (about 2 miles east of Last Stand Hill). It is believed by some historians that he observed the last moments of Custer's death, but he was not able to determine what was happening (too far away), and he was driven away by the approach of large numbers of Indians, forcing him to return to Reno's hilltop position. After the battle, he personally blamed Major Reno for Custer's death. His health failing, he was detailed to recruiting duty at the Cavalry Recruiting Office in New York City, where he suddenly died at age 38. Initially buried December 14, 1876 on Governors Island with full military honors, he was reinterred in the Cypress Hills National Cemetery.