As far as I know, you don't need a license. But, to be on the safe side...
you can actually call the army (as I understand it they assist with other branches as well) and ask questions like these, for free! They can also assist with other things, like where insignia are supposed to go, how they're arranged, uniform standards, etc.
Department of the Army's Office of the Chief of Public Affairs
U.S. Army Office of the Chief of Public Affairs, Western Region
10880 Wilshire Boulevard, Suite 1250
Los Angeles, California 90024
Phone: (310) 235-7621
Fax: (310) 235-6075