The requirements  for military hair grooming standards are necessary
    to maintain good order and discipline. Many haircut styles are
    acceptable by the Army, Navy, Air Force or USMC as long as they are
    clean, neat and conservative.  However, remember that all military
    services have slightly different haircut requirements.

    The most basic military haircut requirements require that sideburns do
    not exceed the ear opening, that the hair be neatly tapered and that
    bulk normally not exceed 2 inches. Further  military haircut standards
    for the Army, Navy, Air Force and USMC are below.
       
Military-Haircut.com
Military Hair Cut Standards, Styles, and requirements