Related Rule
United States of America
Practice Relating to Rule 6. Civilians’ Loss of Protection from Attack
Section A. Direct participation in hostilities
The US Field Manual (1956) states: “Persons who are not members of the armed forces … who bear arms or engage in other conduct hostile to the enemy thereby deprive themselves of many of the privileges attaching to the members of the civilian population”. 
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, § 60.
The manual specifies that persons who are not members of the armed forces, who commit hostile acts such as “sabotage, destruction of communications facilities, intentional misleading of troops by guides [and] liberation of prisoners of war” about or behind enemy lines may be tried and sentenced to execution or imprisonment. 
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, § 81.
The US Air Force Pamphlet (1976) states: “Civilians enjoy the protection afforded by law unless and for such time as they take a direct part in hostilities.” 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 5-3.
The pamphlet further states:
Taking a direct part in hostilities covers acts of war intended by their nature and purpose to strike at enemy personnel and material. Thus a civilian taking part in fighting, whether singly or as a member of a group, loses the immunity given civilians. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 5-3(a).
[emphasis in original]
The US Air Force Commander’s Handbook (1980) states that “anyone who personally tries to kill, injure or capture enemy persons or objects” is liable to attack. The manual adds:
The same would be true of anyone acting as a guard for military activity, as a member of a weapon crew, or as a crewman on a military aircraft in combat … Civilians who collect intelligence information, or otherwise act as part of the enemy’s military intelligence network, are lawful objects of attack. Members of a civilian ground observer corps who report the approach of hostile aircraft would also be taking a direct part in hostilities. The rescue of military airmen downed on land is a combatant activity that is not protected under international law. Civilians engaged in the rescue and return of enemy aircrew members are therefore subject to attack. This would include, for example, members of a civilian air auxiliary, such as the US Civil Air Patrol, who engage in military search and rescue activity in wartime. Note, however, that care of the wounded on land, and the rescue of persons downed at sea or shipwrecked, are protected activities under international law. 
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, § 2-8.
The US Naval Handbook (1995) states:
Civilians who take a direct part in hostilities by taking up arms or otherwise trying to kill, injure, or capture enemy persons or destroy enemy property lose their immunity and may be attacked. Similarly, civilians serving as lookouts, guards, or intelligence agents for military forces may be attacked. Direct participation may also include civilians serving as guards, intelligence agents, or lookouts on behalf of military forces. Direct participation in hostilities must be judged on a case-by-case basis. Combatants in the field must make an honest determination as to whether a particular civilian is or is not subject to deliberate attack based on the person’s behavior, location and attire, and other information available at the time. 
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), § 11.3.
The US Naval Handbook (2007) states:
Unlawful combatants who are not members of forces or parties declared hostile but who are taking a direct part in hostilities may be attacked while they are taking a direct part in hostilities, unless they are hors de combat. Direct participation in hostilities must be judged on a case-by-case basis. Some examples include taking up arms or otherwise trying to kill, injure, or capture enemy personnel or destroy enemy property. Also, civilians serving as lookouts or guards, or intelligence agents for military forces may be considered to be directly participating in hostilities. Combatants in the field must make an honest determination as to whether a particular person is or is not taking a direct part in hostilities based on the person’s behavior, location and attire, and other information available at the time. 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-12.1/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 Homeland Security, US Coast Guard, July 2007, § 8.2.2.
The Handbook also states: “Civilian protection from deliberate attack is contingent on their nonparticipation in hostilities.” 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-12.1/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 Homeland Security, US Coast Guard, July 2007, § 8.3.
The Al-Marri case in June 2007 involved a citizen of Qatar, legally resident in the United States, who had been arrested in December 2001 on terrorism-related charges and confined to a US naval prison in Charleston as an enemy combatant (the US President having determined that he was closely associated with al Qaeda and engaged in conduct in preparation for acts of terrorism). Appearing before the US Court of Appeals for the Fourth Circuit, Al-Marri appealed a decision of a District Court, which had denied his petition for a writ of habeas corpus. In a majority judgment, the Court of Appeals rejected the Government’s arguments that civilians become enemy combatants if they engaged in criminal conduct on behalf of an enemy organization and that the President had inherent constitutional authority to order the military to detain such persons. It reversed the decision of the lower court and remanded the case back to it, with instructions to issue a writ of habeas corpus directing the Secretary of Defense to release Al-Marri from military custody. On matters regarding “direct participation in hostilities”, the Court of Appeals stated:
[W]e recognize that some commentators have suggested that “for such time as they take a direct part in hostilities,” participants in non-international armed conflicts may, as a matter of customary international law, be placed in the formal legal category of “enemy combatant.” See, e.g., Curtis A. Bradley & Jack L. Goldsmith, Congressional Authorization and the War on Terrorism, 118 Harv. L. Rev. 2047, 2115 & n.304 (2005) (internal quotation marks omitted). No precedent from the Supreme Court or this court endorses this view, and the Government itself has not advanced such an argument. This may be because even were a court to follow this approach in some cases, it would not assist the Government here. For the Government has proffered no evidence that al-Marri has taken a “direct part in hostilities.” Moreover, the United States has elsewhere adopted a formal treaty understanding of the meaning of the term “direct part in hostilities,” which plainly excludes al-Marri. See Message from the President of the United States Transmitting Two Optional Protocols to the Convention on the Rights of the Child, S. Treaty Doc. No. 106-37, at VII (2000) (distinguishing between “immediate and actual action on the battlefield” and “indirect participation,” including gathering and transmitting military information, weapons, and supplies). 
United States, Court of Appeals for the Fourth Circuit, Al-Marri case, Judgment, 11 June 2007, § IIIB3.
[emphasis in original]
Having ordered that Al-Marri’s military detention cease in the 11 June 2007 judgment, the US Court of Appeals for the Fourth Circuit subsequently vacated the judgment and held an en banc rehearing on 31 October 2007. In its revised judgment on the case, issued on 15 July 2008, the Court held that Al-Marri could be held in military detention indefinitely as an enemy combatant, stating:
Having considered the briefs and arguments of the parties, the en banc court now holds: (1) by a 5 to 4 vote … that, if the Government’s allegations about al-Marri are true, Congress has empowered the President to detain him as an enemy combatant; and (2) by a 5 to 4 vote … that, assuming Congress has empowered the President to detain al-Marri as an enemy combatant provided the Government’s allegations against him are true, al-Marri has not been afforded sufficient process to challenge his designation as an enemy combatant.
Accordingly, the judgment of the district court is reversed and remanded for further proceedings consistent with the opinions that follow. 
United States, Court of Appeals for the Fourth Circuit, Al-Marri case, Judgment, 15 July 2008, p. 5.
In the Hamlily case in 2009, in which the petitioners challenged the legality of their detention at Guantánamo by seeking writs of habeas corpus, the US District Court for the District of Columbia ruled that the US Government’s detention policy is generally consistent with the authority conferred on the US President under the Authorization for Use of Military Force (AUMF), Public Law 107-40, 115 Stat. 224, 18 September 2001, and core law of war principles that govern non-international armed conflicts. The Court stated in relation to its interpretation of “direct participation in hostilities”:
For purposes of these habeas proceedings, the Court interprets the phrase “committed a belligerent act” to cover any person who has directly participated in hostilities. That conclusion is consistent with the law of war. See [1977] Additional Protocol II, art. 13(3) (stating that civilians shall not be subject to military force “unless and for such time as they take a direct part in hostilities”); [1977] Additional Protocol I, art. 51(3) (same). As the Court has noted above … the precise scope of the phrase “direct participation in hostilities” remains unsettled and the International Committee of the Red Cross is coordinating an effort among experts “to clarify the precise meaning of the notion of ‘direct participation in hostilities’, which has never been defined in treaty law.” ICRC, Report: Direct Participation in Hostilities. In these proceedings, the Court will rely on the settled aspects of the standard. “[L]ittle doubt exists that a civilian carrying out an attack would be directly participating in hostilities. In the same vein, legal experts seem to agree that civilians preparing or returning from combat operations are still considered to be directly participating in hostilities, although precise indication as to when preparation begins and return ends remains controversial.” Id. But any further refinement of the concept of “direct participation” will await examination of particular cases. 
United States, District Court for the District of Columbia, Hamlily case, Judgment, 19 May 2009, pp. 20–21.
In 1987, the Deputy Legal Adviser of the US Department of State affirmed: “We also support the principle … that immunity not be extended to civilians who are taking part in hostilities.” 
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, p. 426.
In 1989, a US memorandum of law concerning the prohibition of assassination stated:
While there is general agreement among law-of-war experts that civilians who participate in hostilities may be regarded as combatants, there is no agreement as to the degree of participation necessary to make an individual civilian a combatant … There is a lack of agreement on this matter, and no existing law-of-war treaty provides clarification or assistance. Historically, however, the decision as to the level at which civilians may be regarded as combatants or “quasi-combatants” and thereby subject to attack generally has been policy rather than a legal matter. The technological revolution in warfare that has occurred over the past two centuries has resulted in a joining of segments of the civilian population with each nation’s conduct of military operations and vital support activities … Finally, one rule of thumb with regard to the likelihood that an individual may be subject to lawful attack is his (or her) immunity from military service if continued service in his (or her) civilian position is of greater value to a nation’s war effort than that person’s service in the military. A prime example would be civilian scientists occupying key positions in a weapons program regarded as vital to a nation’s national security or war aims. Thus, more than 90% of the World War II Project Manhattan personnel were civilians, and their participation in the U.S. atomic weapons program was of such importance as to have made them liable to legitimate attack. Similarly, the September 1944 Allied bombing raids on the German rocket sites at Peenemunde regarded the death of scientists involved in research and development at that facility to have been as important as destruction of the missiles themselves. 
United States, Department of the Army, Office of the Judge Advocate General, Memorandum of Law: Executive Order 12333 and Assassination, 2 November 1989, The Army Lawyer, Pamphlet 27-50-204, December 1989.
In 1992, in its final report to Congress on the conduct of the Gulf War, the US Department of Defense stated: “As a general principle, the law of war prohibits … the direct, intentional attack of civilians not taking part in hostilities.” 
United States, Department of Defense, Final Report to Congress on the Conduct of the Persian Gulf War, 10 April 1992, Appendix O, The Role of the Law of War, ILM, Vol. 31, 1992, p. 622.
The Report on US Practice states:
Under the practice of the United States, civilians lose immunity from direct attack if, and for so long as, they are committing hostile acts or otherwise taking a direct part in hostilities. These conditions may be met by bearing arms or by aiding the enemy with arms, ammunition, supplies, money or intelligence information or even by holding unauthorized intercourse with enemy personnel. Other acts might be considered to be taking a direct part in hostilities, depending on the intensity of the conflict and other circumstances. 
Report on US Practice, 1997, Chapter 1.2.