Like the American Revolution, some women dressed as men in order to serve in the Civil War. Post-war estimates put the number at about 400, but even at the time Mary Livermore with the U.S. Sanitary Commission wrote:
I am convinced that a larger number of women disguised themselves and enlisted in the service, for one cause or other, than was dreamed of. Entrenched in secrecy, and regarded as men, they were sometimes revealed as women, by accident or casualty. Some startling histories of these military women were current in the gossip of army life.
One of those women was Frances Clayton, who dressed as a man and served many months in the Missouri artillery and cavalry units. The image comes from the Trustees of the Boston Public Library and the National Archives.
Hinges of History 