There are many artists who have created art as part of their military assignment or civilian artists who have been embeded to create original art over the years. Winslow Homer for instance was an artist/illustrator who gave us very detailed images of the Civil War.

Take a look at these links which represent only a partial reference of what you can find about military art.

Art of the American Soldier-Why paint military scenes in the age of Photography?

http://www.youtube.com/watch?v=H1D_zFJOnlY

Army Museums

http://www.history.army.mil/html/museums/dir-links.html

The Art of James Dietz

(You can see original works of art by this artist at Fort Irwin Museum)

http://www.jamesdietz.com/