You can be identified as a security treat and shot at if you enter the pentagon without permission. The pentagon is the headquarter of the United States Department of Defense thus a restricted area. Any trespassers found in it without permission, are prosecuted.
The basement of the pentagon is actually a mall with department stores, shops, etc. In order to pass through the section where employees only are allowed you must go through an entrance with guards similar to an airport. They do have guns, but I don't know under what circumstances they will shoot.