Rule 3. Definition of Combatants
Rule 3. All members of the armed forces of a party to the conflict are combatants, except medical and religious personnel.
Summary
State practice establishes this rule as a norm of customary international law in international armed conflicts. For purposes of the principle of distinction (see Rule 1), members of State armed forces may be considered combatants in both international and non-international armed conflicts. Combatant status, on the other hand, exists only in international armed conflicts (see introductory note to Chapter 33).
International armed conflicts
This rule goes back to the Hague Regulations, according to which “the armed forces of the belligerent parties may consist of combatants and non-combatants”.[1]  It is now set forth in Article 43(2) of Additional Protocol I.[2] 
Numerous military manuals contain this definition of combatants.[3]  It is supported by official statements and reported practice.[4]  This practice includes that of States not, or not at the time, party to Additional Protocol I.[5] 
No official contrary practice was found.
Non-international armed conflicts
Common Article 3 of the Geneva Conventions and Additional Protocol II refer to “armed forces” and Additional Protocol II also to “dissident armed forces and other organized armed groups”. These concepts are not further defined in the practice pertaining to non-international armed conflicts. While State armed forces may be considered combatants for purposes of the principle of distinction (see Rule 1), practice is not clear as to the situation of members of armed opposition groups. Practice does indicate, however, that persons do not enjoy the protection against attack accorded to civilians when they take a direct part in hostilities (see Rule 6).
Persons taking a direct part in hostilities in non-international armed conflicts are sometimes labelled “combatants”. For example, in a resolution on respect for human rights in armed conflict adopted in 1970, the UN General Assembly speaks of “combatants in all armed conflicts”.[6]  More recently, the term “combatant” was used in the Cairo Declaration and Cairo Plan of Action for both types of conflicts.[7]  However, this designation is only used in its generic meaning and indicates that these persons do not enjoy the protection against attack accorded to civilians, but this does not imply a right to combatant status or prisoner-of-war status, as applicable in international armed conflicts (see Chapter 33). The lawfulness of direct participation in hostilities in non-international armed conflicts is governed by national law. While such persons could also be called “fighters”, this term would be translated as “combatant” in a number of languages and is therefore not wholly satisfactory either.
Treaty provisions use different designations that can apply to “fighters” in the context of non-international armed conflicts, including: persons taking active part in the hostilities;[8]  members of dissident armed forces or other organized armed groups;[9]  persons who take a direct part in hostilities;[10]  civilians who take a direct part in hostilities;[11]  civilians taking direct part in hostilities;[12]  and combatant adversary.[13]  The uncertainty about the qualification of members of armed opposition groups is further addressed in the commentaries to Rules 5 and 6.
Interpretation
According to this rule, when military medical and religious personnel are members of the armed forces, they are nevertheless considered non-combatants. According to the First Geneva Convention, temporary medical personnel have to be respected and protected as non-combatants only as long as the medical assignment lasts (see commentary to Rule 25).[14]  As is the case for civilians (see Rule 6), respect for non-combatants is contingent on their abstaining from taking a direct part in hostilities.
The military manuals of Germany and the United States point out that there can be other non-combatant members of the armed forces besides medical and religious personnel. Germany’s Military Manual explains that “combatants are persons who may take a direct part in hostilities, i.e., participate in the use of a weapon or a weapon-system in an indispensable function”, and specifies, therefore, that “persons who are members of the armed forces but do not have any combat mission, such as judges, government officials and blue-collar workers, are non-combatants”.[15]  The US Naval Handbook states that “civil defense personnel and members of the armed forces who have acquired civil defense status” are non-combatants, in addition to medical and religious personnel.[16] 
Non-combatant members of the armed forces are not to be confused, however, with civilians accompanying armed forces who are not members of the armed forces by definition.[17] 
While in some countries, entire segments of the population between certain ages may be drafted into the armed forces in the event of armed conflict, only those persons who are actually drafted, i.e., who are actually incorporated into the armed forces, can be considered combatants. Potential mobilization does not render the person concerned a combatant liable to attack.[18] 

[1] Hague Regulations, Article 3 (cited in Vol. II, Ch. 1, § 571).
[2] Additional Protocol I, Article 43(2) (adopted by consensus) (ibid., § 572).
[3] See, e.g., the military manuals of Argentina (ibid., § 574), Australia (ibid., § 575), Belgium (ibid., § 576), Benin (ibid., § 577), Cameroon (ibid., § 578), Canada (ibid., § 579), Colombia (ibid., § 580), Croatia (ibid., §§ 581–582), Dominican Republic (ibid., § 583), Ecuador (ibid., § 584), France (ibid., §§ 585–586), Germany (ibid., § 587), Hungary (ibid., § 588), Indonesia (ibid., § 589), Israel (ibid., § 590), Italy (ibid., §§ 591–592), Kenya (ibid., § 593), South Korea (ibid., § 594), Madagascar (ibid., § 595), Netherlands (ibid., § 596), New Zealand (ibid., § 597), Russia (ibid., § 598), South Africa (ibid., § 599), Spain (ibid., § 600), Sweden (ibid., § 601), Togo (ibid., § 602), United Kingdom (ibid., § 603) and United States (ibid., §§ 604–606).
[4] See, e.g., the practice of Argentina (ibid., 611), India (ibid., § 612), Iraq (ibid., § 613), Japan (ibid., § 614), Jordan (ibid., § 615) and Syria (ibid., § 619).
[5] See, e.g., the practice of France (ibid., § 585), Indonesia (ibid., § 589), Israel (ibid., § 590), Kenya (ibid., § 593), United Kingdom (ibid., § 603) and United States (ibid., §§ 604–606).
[6] UN General Assembly, Res. 2676 (XXV), 9 December 1970, preamble and § 5.
[7] Cairo Declaration, Sections 68–69, and Cairo Plan of Action, Section 82, both adopted at the Africa-Europe Summit held under the Aegis of the Organization of African Unity and the European Union, 3–4 April 2000.
[8] Geneva Conventions, common Article 3.
[9] Additional Protocol II, Article 1(1) (adopted by 58 votes in favour, 5 against and 29 abstentions) (cited in Vol. II, Ch. 1, § 633).
[10] Additional Protocol II, Article 4(1) (adopted by consensus).
[11] Additional Protocol II, Article 13(3) (adopted by consensus) (ibid., § 756).
[12] ICC Statute, Article 8(2)(e)(i).
[13] ICC Statute, Article 8(2)(e)(ix).
[14] First Geneva Convention, Article 25 (cited in Vol. II, Ch. 7, § 7).
[15] Germany, Military Manual (cited in Vol. II, Ch. 1, § 587).
[16] United States, Naval Handbook (ibid., § 605).
[17] See Third Geneva Convention, Article 4(A)(4).
[18] This conclusion is based on discussions during the second consultation with academic and governmental experts in the framework of this study in May 1999 and the general agreement among the experts to this effect. The experts also considered that it may be necessary to consider the legislation of a State in determining when reservists actually become members of the armed forces.