The film states that the Civil War ended with Lee's surrender of his army at Appomattox on April 9, 1865. In actuality, there were still several other Confederate armies in the field and fighting continued until the last one surrendered in May 1865.
The uniform Driscoll was wearing when he was shot by O'Meara at Appomattox was not the type worn by Union cavalry during the Civil War. It was the type of uniform worn by the U.S. Cavalry in the Indian Wars of the 1870's and 1880's.
The bulk of the film is in a very arid desert area, not the landscape in Sioux territory.