Related Rule
United States of America
Practice Relating to Rule 108. Mercenaries
The US Air Force Commander’s Handbook (1980) states:
Until recently, there was no generally accepted definition of a “mercenary,” but the term was usually applied to foreigners who took part in an armed conflict on one side or the other, primarily for high pay or hope of booty … The definition of “mercenary” in [the 1977 Additional Protocol I] is so narrow that few persons would fit within it. The United States has signed this Protocol but has not yet ratified it. 
United States, Air Force Pamphlet 110-34, Commander’s Handbook on the Law of Armed Conflict, Judge Advocate General, US Department of the Air Force, 25 July 1980, § 5-3.
In 1987, the Deputy Legal Adviser of the US Department of State affirmed:
We do not favor the provisions of article 47 on mercenaries, which among other things introduce political factors that do not belong in international humanitarian law, and do not consider the provisions of article 47 to be part of current customary law. 
United States, Remarks of Michael J. Matheson, Deputy Legal Adviser, US Department of State, The Sixth Annual American Red Cross-Washington College of Law Conference on International Humanitarian Law: A Workshop on Customary International Law and the 1977 Protocols Additional to the 1949 Geneva Conventions, American University Journal of International Law and Policy, Vol. 2, 1987, pp. 426–427.
The US Air Force Commander’s Handbook (1980) states:
In recent years, many countries have claimed that “mercenaries” are unlawful combatants and subject to punishment upon capture …
a. The United States has long recognized that neutral nationals taking part in an armed conflict can encourage the escalation of that conflict, and US statutes place certain limits on the recruitment of mercenaries in this country. We have also, however, regarded mercenaries as lawful combatants entitled to PW status upon capture. The US government has always protested vigorously against any attempt by other nations to punish American citizens as mercenaries.
b. [The 1977 Additional Protocol I] provides that mercenaries do not have the right to be combatants or prisoners of war, but the definition of “mercenary” in this Protocol is so narrow that few persons would fit within it. 
United States, Air Force Pamphlet 110-34, Commander’s Handbook on the Law of Armed Conflict, Judge Advocate General, US Department of the Air Force, 25 July 1980, § 5-3.
In 1980, in a memorandum concerning the international legal rights of captured mercenaries, the US Department of State stated:
The act of being a mercenary is not a crime under international law. An individual who is accused of being a mercenary and who is captured during an armed conflict is entitled to the basic humanitarian protections of the international law applicable in armed conflict, including those specified in the Geneva Convention Relative to the Protection of Civilian Persons in Time of War. The specific rights which such an individual would be entitled to vary depending on whether the conflict is an international conflict or an internal one and, in the case of international armed conflicts, on whether the person is entitled to prisoner-of-war status … The protections of [common] article 3 [of the 1949 Geneva Conventions] would also apply to any captured individual accused of being a mercenary during a civil war. [Common Article 3] does not provide any immunity from prosecution to individuals for engaging in combatant acts. The provisions of the Geneva Conventions dealing with prisoners of war do not apply in civil wars, and combatants captured during civil wars are not prisoners of war within the meaning of international law. 
United States, International Legal Rights of Captured Mercenaries, Memorandum prepared by the Attorney-Adviser in the Office of the Assistant Legal Adviser for African Affairs, US Department of State, 17 October 1980, reprinted in Marian Nash (Leich), Cumulative Digest of United States Practice in International Law, 1981-1988, Department of State Publication 10120, Washington, D.C., 1993–1995, pp. 3457 and 3463–3464.
In 1987, the Deputy Legal Adviser of the US Department of State affirmed:
We do not favor the provisions of article 47 on mercenaries, which among other things introduce political factors that do not belong in international humanitarian law, and do not consider the provisions of article 47 to be part of current customary law. 
United States, Remarks of Michael J. Matheson, Deputy Legal Adviser, US Department of State, The Sixth Annual American Red Cross-Washington College of Law Conference on International Humanitarian Law: A Workshop on Customary International Law and the 1977 Protocols Additional to the 1949 Geneva Conventions, American University Journal of International Law and Policy, Vol. 2, 1987, pp. 426–427.
In 1987, the Legal Adviser of the US Department of State stated:
For a third example [of why the Joint Chiefs of Staff judged the 1977 Additional Protocol I too ambiguous and complicated to use as a practical guide for military operations], article 47 of Protocol I provides that “a mercenary shall not have the right to be a combatant or a prisoner of war.” This article was included in the Protocol not for humanitarian reasons, but purely to make the political point that mercenary activity in the Third World is unwelcome. In doing so, this article disregards one of the fundamental principles of international humanitarian law by defining the right to combatant status, at least in part, on the basis of the personal or political motivations of the individual in question. This politicizing of the rules of warfare is contrary to Western interests and the interests of humanitarian law itself. 
United States, Remarks of Judge Abraham D. Sofaer, Legal Adviser, US Department of State, The Sixth Annual American Red Cross-Washington College of Law Conference on International Humanitarian Law: A Workshop on Customary International Law and the 1977 Protocols Additional to the 1949 Geneva Conventions, American University Journal of International Law and Policy, Vol. 2, 1987, p. 469.