Rule 4. Definition of Armed Forces
Rule 4. The armed forces of a party to the conflict consist of all organized armed forces, groups and units which are under a command responsible to that party for the conduct of its subordinates.
Summary
State practice establishes this rule as a norm of customary international law applicable in international armed conflicts. For purposes of the principle of distinction, it may also apply to State armed forces in non-international armed conflicts.[1] 
International armed conflicts
This rule is set forth in Article 43(1) of Additional Protocol I.[2] 
Many military manuals specify that the armed forces of a party to the conflict consist of all organized armed groups which are under a command responsible to that party for the conduct of its subordinates.[3]  This definition is supported by official statements and reported practice.[4]  Practice includes that of States not, or not at the time, party to Additional Protocol I.[5] 
In essence, this definition of armed forces covers all persons who fight on behalf of a party to a conflict and who subordinate themselves to its command. As a result, a combatant is any person who, under responsible command, engages in hostile acts in an armed conflict on behalf of a party to the conflict. The conditions imposed on armed forces vest in the group as such. The members of such armed forces are liable to attack.
This definition of armed forces builds upon earlier definitions contained in the Hague Regulations and the Third Geneva Convention which sought to determine who are combatants entitled to prisoner-of-war status. Article 1 of the Hague Regulations provides that the laws, rights and duties of war apply not only to armies, but also to militia and volunteer corps fulfilling four conditions:
It further specifies that in countries where militia or volunteer corps (so-called “irregular” armed forces) constitute the army, or form part of it, they are included under the denomination “army”.[6]  This definition is also used in Article 4 of the Third Geneva Convention, with the addition of organized resistance movements.[7]  The Hague Regulations and the Third Geneva Convention thus consider all members of armed forces to be combatants and require militia and volunteer corps, including organized resistance movements, to comply with four conditions in order for them to be considered combatants entitled to prisoner-of-war status. The idea underlying these definitions is that the regular armed forces fulfil these four conditions per se and, as a result, they are not explicitly enumerated with respect to them. The definition contained in Additional Protocol I does not distinguish between the regular armed forces and other armed groups or units, but defines all armed forces, groups and units which are under a command responsible to a party for the conduct of its subordinates as armed forces of that party. Both definitions express the same idea, namely that all persons who fight in the name of a party to a conflict – who “belong to” a party in the words of Article 4 of the Third Geneva Convention – are combatants. The four conditions contained in the Hague Regulations and the Third Geneva Convention have been reduced to two conditions, the main difference being the exclusion of the requirements of visibility for the definition of armed forces as such. The requirement of visibility is relevant with respect to a combatant’s entitlement to prisoner-of-war status (see Rule 106). Additional Protocol I, therefore, has lifted this requirement from the definition of armed forces (Article 43) and placed it in the provision dealing with combatants and prisoner-of-war status (Article 44).
In addition, Article 43 of Additional Protocol I does not mention the requirement to respect the laws and customs of war but includes a requirement to have an internal disciplinary system to enforce compliance with international humanitarian law, but this change does not substantially alter the definition of armed forces for the purposes of determining those combatants entitled to prisoner-of-war status. The requirement of an internal disciplinary system supplements the provisions concerning command responsibility (see Rules 152–153) and is a corollary of the obligation to issue instructions which comply with international humanitarian law (see commentary to Rule 139).[8] 
Articles 43 and 44 of Additional Protocol I reaffirm what was already stated in Article 85 of the Third Geneva Convention, namely that “prisoners of war prosecuted under the laws of the Detaining Power for acts committed prior to capture shall retain, even if convicted, the benefits of the present Convention”, that is to say that they retain their status. These provisions “thus preclude any attempt to deny prisoner of war status to members of independent or regular armed forces on the allegation that their force does not enforce some provision of customary or conventional law of armed conflict (as construed by the Detaining Power)”.[9]  Only the failure to distinguish oneself from the civilian population (see Rule 106) or being caught as a spy (see Rule 107) or a mercenary (see Rule 108) warrant forfeiture of prisoner-of-war status.
The definition in Article 43 of Additional Protocol I is now generally applied to all forms of armed groups who belong to a party to an armed conflict to determine whether they constitute armed forces. It is therefore no longer necessary to distinguish between regular and irregular armed forces. All those fulfilling the conditions in Article 43 of Additional Protocol I are armed forces.
Incorporation of paramilitary or armed law enforcement agencies into armed forces
Specific practice was found concerning the incorporation of paramilitary or armed law enforcement agencies, such as police forces, gendarmerie and constabulary, into armed forces.[10]  Examples of such paramilitary agencies incorporated into the armed forces of a State include the Special Auxiliary Force attached to Bishop Muzorewa’s United African National Congress in Zimbabwe, which was integrated into the national army after the Bishop became Prime Minister, and India’s Border Security Force in Assam.[11]  Examples of armed law enforcement agencies being incorporated into the armed forces include the Philippine Constabulary and Spain’s Guardia Civil.[12] 
Incorporation of paramilitary or armed law enforcement agencies into armed forces is usually carried out through a formal act, for example, an act of parliament. In the absence of formal incorporation, the status of such groups will be judged on the facts and in the light of the criteria for defining armed forces. When these units take part in hostilities and fulfil the criteria of armed forces, they are considered combatants. In addition, Additional Protocol I requires a party to the conflict to notify such incorporation to the other parties to the conflict.[13]  Belgium and France issued a general notification to this effect to all States party upon ratification of Additional Protocol I.[14]  This method of satisfying the requirement of notification was explicitly recognized by the Rapporteur of the Working Group at the Diplomatic Conference leading to the adoption of the Additional Protocols.[15]  In the light of the general obligation to distinguish between combatants and civilians (see Rule 1), such notification is important because members of the armed forces of each side have to know who is a member of the armed forces and who is a civilian. Confusion is particularly likely since police forces and gendarmerie usually carry arms and wear a uniform, although in principle their uniforms are not the same as those of the armed forces proper. While notification is not constitutive of the status of the units concerned, it does serve to avoid confusion and thus enhances respect for the principle of distinction.

[1] See CDDH, Official Records, Vol. X, CDDH/I/238/Rev. 1, pp. 93–94; see also Yves Sandoz, Christophe Swinarski, Bruno Zimmermann (eds.), Commentary on the Additional Protocols, ICRC, Geneva, 1987, § 4462.
[2] Additional Protocol I, Article 43(1) (adopted by consensus) (cited in Vol. II, Ch. 1, § 631).
[3] See, e.g., the military manuals of Argentina (ibid., § 637), Australia (ibid., § 638), Canada (ibid., § 642), Croatia (ibid., § 644), Germany (ibid., § 647), Hungary (ibid., § 648), Italy (ibid., § 651), Kenya (ibid., § 652), Netherlands (ibid., § 654), New Zealand (ibid., § 655), Nigeria (ibid., § 656), Russia (ibid., § 657), Spain (ibid., § 659), Sweden (ibid., § 660) and United Kingdom (ibid., § 662).
[4] See, e.g., the practice of Belgium (ibid., § 670), France (ibid., § 671), Germany (ibid., § 672), Iran (ibid., § 673), Netherlands (ibid., § 676) and Syria (ibid., § 677).
[5] See, e.g., the practice of France (ibid., § 671), Kenya (ibid., § 652), United Kingdom (ibid., § 662) and United States (ibid., § 665).
[6] Hague Regulations, Article 1 (ibid., § 628).
[7] Third Geneva Convention, Article 4 (ibid., § 630).
[8] See Yves Sandoz, Christophe Swinarski, Bruno Zimmermann (eds.), Commentary on the Additional Protocols, ICRC, Geneva, 1987, § 1675.
[9] Michael Bothe, Karl Josef Partsch, Waldemar A. Solf, New Rules for Victims of Armed Conflicts, Martinus Nijhoff, The Hague, 1982, p. 239.
[10] See, e.g., the practice of Argentina (cited in Vol. II, Ch. 1, § 688), Belgium (ibid., § 685), Canada (ibid., § 689), France (ibid., § 686), Germany (ibid., § 690), Netherlands (ibid., § 691), New Zealand (ibid., § 692), Spain (ibid., §§ 693 and 696) and Philippines (ibid., § 695) and the reported practice of India (ibid., § 698), South Korea (ibid., § 699), Syria (ibid., § 700) and Zimbabwe (ibid., § 697).
[11] These examples are quoted in New Zealand, Military Manual (ibid., § 692).
[12] Philippines, Decree on the Constitution of the Integrated National Police (ibid., § 695); Spain, Military Criminal Code (ibid., § 696).
[13] Additional Protocol I, Article 43(3) (adopted by consensus) (ibid., § 684).
[14] Belgium, Interpretative declarations made upon ratification of Additional Protocol I (ibid., § 685); France, Reservations and declarations made upon ratification of Additional Protocol I (ibid., § 686).
[15] Yves Sandoz, Christophe Swinarski, Bruno Zimmermann (eds.), Commentary on the Additional Protocols, ICRC, Geneva, 1987, § 1682.