Thomas Bailey Marquis (December 19, 1869 – March 22, 1935) was an American self-taught historian and ethnographer who wrote about the Plains Indians and other subjects of the American frontier. He had a special interest in the destruction of George Armstrong Custer's battalion at the Battle of the Little Bighorn, which became his lifelong obsession. Marquis' body of work is valued by historians for his recording of the life stories of several Plains Indians and his writing on their way of life, particularly those involved in the Custer fight, notably Wooden Leg in A Warrior Who Fought Custer. Marquis carried out this research at a time when few were interested in the Indian version of events, even though no American soldiers survived the Custer fight. Marquis' work is thus both unique and unrepeatable.
Marquis developed his own theories regarding the history of the Cheyenne. One idea in particular, that many of Custer's men committed suicide when the situation became hopeless, proved to be highly controversial. This idea first surfaced in the Wooden Leg narrative, but was most fully developed in Keep the Last Bullet for Yourself, considered by Marquis to be his most important work and the culmination of his Custer research. The latter was not published during Marquis' life; much of his work did not appear in print until the 1970s. The last book to be published, The Cheyennes of Montana (pub. 1978), is regarded as especially valuable by historians. In 2006 a collection of his photographs was published as, A Northern Cheyenne Album.
Marquis was born in Missouri and trained for a career as a printworker. He moved to Montana because he wanted an adventure in the West, but ended up staying there for the remainder of his life. At first he worked as an itinerant printer, but after getting married he trained in medicine and became a physician. The idea was to have a more settled lifestyle, especially after his first daughter was born, but Marquis seemed to have a wanderlust, frequently changing location and often considering a change of career (he qualified as an attorney at one point). This became especially apparent after his military service in World War I and the failure of his marriage. These moves were always within Montana, and his final location was in Hardin on the Crow Reservation close to the Custer battle site. He founded a museum there that has now been incorporated into the Little Bighorn Battlefield National Monument.
Marquis first took up writing around the turn of the century, writing newspaper articles. In 1922 he was appointed agency physician to the Northern Cheyenne. He resigned after only ten weeks, but he was there long enough to realize that several participants of the Custer battle were still alive. Between 1923 and 1926 Marquis practiced medicine privately on the Crow Reservation but then tried to make writing his permanent career. Throughout his life Marquis was often in debt; his writing was sometimes interrupted by the need to earn money at medicine, or even his original career of typesetting. From 1926 he returned to his contacts among the Cheyenne, interviewing them in depth. The trust he developed with them allowed them to open up to him in a way they had not with any other outsiders. The result formed the bulk of Marquis' most important works.