Related Rule
United States of America
Practice Relating to Rule 4. Definition of Armed Forces
The US Field Manual (1956) and Air Force Pamphlet (1976) define armed forces with reference to Article 4(A) of the 1949 Geneva Convention III. 
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, § 61; Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 3-2.
The US Naval Handbook (1995) states that combatants
include all members of the regularly organized armed forces of a party to the conflict … as well as irregular forces who are under responsible command and subject to internal military discipline, carry their arms openly, and otherwise distinguish themselves clearly from the civilian population. 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-2.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Transportation, US Coast Guard, October 1995 (formerly NWP 9 (Rev. A)/FMFM 1-10, October 1989), § 5.3.