In July 1855, six companies from the 2nd Infantry rook possession of an old fur trading post on the banks of the Upper Missouri River and transformed it into a base of operations against the Sioux. But before setting out on this assignment, the officers and men of this regiment spent almost a year and a half at Carlisle Barracks filling their ranks, drilling, and preparing for service on the prairie. Among the officers in this contingent was 34-year-old Lieutenant Thomas William Sweeny.
A native of County Cork, Ireland, Sweeny and his family came to the United States in 1832 and settled in New York, where he received his education and later apprenticed to a book publisher. Sweeny also belonged to "military and literary" clubs in the city, and it was through these associations that he received a commission in the 1st New York Volunteers when the Mexican War started. The regiment took part in the drive from Veracruz to Mexico City, and suffered heavy casualties in the battle of Churubusco. Among those casualties was Lieutenant Sweeny, whose right arm was amputated. After the war he received a commission in the 2nd Infantry and served at San Diego and Fort Yuma in Southern California from 1849 to 1853. He returned to New York in January 1854, and after a brief stint on recruiting duty joined his regiment at Carlisle Barracks in September of that year.