Thomas Mower McDougall was an officer in the United States Army. The salient point in his military career occurred when he took part in the Battle of the Little Bighorn, surviving because he and his unit was not with George Armstrong Custer and the main body of the 7th Cavalry Regiment. Early on the day of battle, McDougall's Company B was assigned to escort the regiment's pack train, but the mules were not used to carrying packs and lagged far behind the other three detachments under Custer, Reno, and Benteen as they went into combat. After viewing the Indian village, and being surprised by its size, Custer sent two urgent orders to bring the mules with the ammunition packs to his detachment of five companies, but by the time these messengers reached Captain McDougall the distance between the pack train and Custer made this order difficult if not impossible to comply with, though a debate on this topic remains to this day.

The Battle of the Little Bighorn, known to the Lakota and other Plains Indians as the Battle of the Greasy Grass, and commonly referred to as Custer's Last Stand, was an armed engagement between combined forces of the Lakota Sioux, Northern Cheyenne, and Arapaho tribes and the 7th Cavalry Regiment of the United States Army. The battle, which resulted in the defeat of U.S. forces, was the most significant action of the Great Sioux War of 1876. It took place on June 25–26, 1876, along the Little Bighorn River in the Crow Indian Reservation in southeastern Montana Territory.