Hays Parks and Direct Participation in Hostilities

by | Oct 7, 2021

Hays Parks Direct Participation

As he did with so many subjects of the law of war, W. Hays Parks left an indelible imprint on the long fraught question of direct participation in hostilities by civilians. From early legal advice to expert consultations and seminal academic contributions, his work significantly shaped current thought and legal doctrine on the subject. The characteristic legal rigor and unwavering military operational focus he brought to the issue were renowned and should continue to inform interpretations of this vital aspect of the law of war.

Direct Participation in Hostilities

At its essence, the direct participation standard is a refinement of the principle of distinction and more immediately of civilian status under the law of war. While civilian status ordinarily offers legal protection from direct attack, civilians who take direct part in hostilities lose that protection. They may be intentionally attacked for the duration of their acts. Unlike the combatant-civilian distinction that resorts to legal status to withhold or assign protection, the direct participation standard is conduct-based.

Civilians who take direct part in hostilities do not lose their civilian status. Rather, their conduct deprives them of protection from attack. A widely accepted expression of the customary and conventional conduct-based standard for lethal targeting of civilians indicates that they enjoy protection from intentional attack, “unless and for such time as they take direct part in hostilities.” It is a standard applicable to both international and non-international armed conflict.

Early Work on Active and Direct Participation

Parks’s early work on the question of conduct-based targeting came in response to expanded use of civilian contractors in contingency operations and armed conflict. Contractors had long been a regular feature of war. However, by the late 1990s and early-to-mid 2000s contractors increasingly performed functions traditionally assigned to armed forces such as base security, military intelligence functions, and forward maintenance of sophisticated military equipment. Parks considered, among other questions, whether contractors would be subject to intentional attack considering their activities and anticipated conduct.

Parks’s legal advice helpfully identified the forces behind the conduct-based standard of targeting. Mid-twentieth century mechanization of warfare, aerial bombing, and resulting expansions of battlefields fostered civilian participation in hostilities and spawned the military necessity to counter their contributions. Additionally, insurgencies and wars of liberation that featured participation by civilians as either full-time or occasional fighters—the so-called “farmer by day, fighter by night” scenario—also fueled demand for a law-of-war response. Meanwhile, modern remote technologies of war, such as aerial drones, that extended targeting capabilities to previously inaccessible areas where civilians made important contributions to hostilities highlighted the need for an effective, conduct-based standard for lawful targeting.

Turning to legal issues, Parks’s early work framed the question of civilian contractors’ protection as a matter of the law of war principle of distinction.[1] Well-steeped in the history of the law of war, he showcased the 1863 U.S. Army General Order 100 (also known as the Lieber Code) as mandating only “protection of the inoffensive citizen of the hostile country” (emphasis added). From that lineage, he assessed the modern standard of civilian protection from intentional attack as contingent on not “taking an active part in the hostilities” (emphasis added).

Parks declined to identify a bright line separating active from inactive participation. However, he identified on one hand civilians performing base maintenance functions, such as trash collection and water supply, as clearly not involving active participation. On the other hand, he observed, “A civilian entering the theater of operations in support or operation of sensitive, high value equipment, such as a weapon system, may be at risk of intentional attack because of the importance of his or her duties.” He added that this determination should usually be made from the perspective of the attacker. That is, whether participation in hostilities is sufficient to divest a civilian of protection depends in large part on the significance of their activities to the enemy force.

Parks’s treatment of the question is notable in several respects. First, he resorted to the term “active” rather than “direct” as the operative standard. Although he did not explain the choice in his early work, he presumably drew the term from the 1949 Geneva Conventions’ resort to the phrase “taking no active part in the hostilities,” to describe persons protected during non-international armed conflict. Second, Parks’s resort to the term “active” led him to focus on the nature and importance of the civilian participation in question rather than on the proximate effects or the degree of connection to harm or violence that a determination of directness or indirectness might involve.

Parks later adopted the “direct part” terminology. However, he seemed to do so only by equating the terms “active” and “direct” as synonymous for purposes of delineating the conduct-based targeting standard as others have since done including scholars and tribunals. And he persisted in the view that military importance or significance of participation was more relevant to conduct-based targeting than directness or proximity of civilian conduct to resulting harm.

Parks and the Interpretive Guidance

Parks’s most famous contribution to the direct participation question, however, may be his objection to the International Committee of the Red Cross Interpretive Guidance on the Notion of Direct Participation in Hostilities. The outcome of a series of expert consultations from 2003 to 2008 hosted by the International Committee of the Red Cross and the Hague-based T.M.C. Asser Institute, the Interpretive Guidance sought to clarify interpretation and guide implementation of the notion of direct participation in hostilities. Although Parks participated in most stages of the expert process, he ultimately withdrew along with many other experts and withheld his support from the Interpretive Guidance.

Most notably, Parks did not support additional restrictions on conduct-based targeting found in Part IX of the Interpretive Guidance. Reportedly a late and unanticipated addition to the project, Part IX of the Interpretive Guidance asserted that, beyond other law of war considerations, the “kind and degree of force” used against persons who may be lawfully targeted “must not exceed what is actually necessary.”

Where, classically, the conduct-based standard regarded the military necessity of lethal attack as presumed or “baked into” direct participation (and for that matter into combatant status), Part IX required a further evaluation of military necessity. In addition to establishing combatant status or direct participation in hostilities by a civilian, under Part IX an attacker must separately determine that the overall circumstances absolutely require resort to lethal force. Where less-than-lethal force, such as capture or other means would achieve military purposes, lethal force could not be used against such persons, notwithstanding their status or conduct.

Parks published his dissent to Part IX with characteristic vigor, supported by exhaustive research in an article that accompanied other views on the Interpretive Guidance. He marshaled a host of procedural, historical, legal, and operational counterarguments.

Parks first emphasized that the direct participation question derives from the law of war principle of discrimination. The obligation in attack to discriminate between civilians and combatants could only work, Parks argued, if paired with “a concomitant obligation on the part of an individual civilian not to use his or her protected status to engage in hostile acts . . . ” (pp. 772-73). His legal points also emphasized the role of the law of war as a lex specialis for regulation of attacks during armed conflict. He detected in Part IX the influence of human rights law and law enforcement regulations on the use of force. On the question of which persons may or may not be targeted, he argued the law of war left no room for rules derived from other legal regimes.

Second, Parks objected to academic work that influenced Part IX notions of military necessity. An influential study of targeted killing had advocated resort to a definition of military objectives in the 1977 Additional Protocol I to the 1949 Geneva Conventions (AP I) to determine the degree and kind of force that may be lawfully used against persons. Article 52(2) of AP I states,

In so far as objects are concerned, military objectives are limited to those objects which by their nature, location, purpose or use make an effective contribution to military action and whose total or partial destruction, capture or neutralization, in the circumstances ruling at the time, offers a definite military advantage.

Applied to attacks on persons, Article 52(2) would require, beyond a determination of qualifying status or conduct, that lethal force was required to achieve a definite military advantage. Accordingly, Part IX advocated resort to a graduated consideration of force from capture, to less-than-lethal means, to lethal force.

Parks’s article insisted that, consistent with its plain language, Article 52(2) was strictly limited to objects and had no role in regulating attacks on persons. In Parks’s view, either a status or conduct determination alone was sufficient to justify intentional attack against persons. It appears Parks’s arguments at the ICRC-Asser process influenced the final, published version of the Interpretive Guidance. Although Part IX counsels consideration of less-than-lethal military action when determining “the kind and degree of force,” it includes no reference to Article 52(2) requirements apparently included in prior drafts of Part IX.

Finally, Parks reframed interpretation of the direct participation standard as a question of military operational reality. Parks applied his extensive knowledge of ballistics and wound science to argue that a legal regime that required consideration or application of a graduated continuum of force would be impractical in armed conflict. He argued that even means presumed to be lethal, such as high-velocity military rounds, often proved to be less than lethal in reality and that overwhelming force is most frequently required to reliably render persons hors de combat. In combat, the deliberation and hesitation to use overwhelmingly lethal force counseled by Part IX, he argued, would itself prove lethal to the attacker.

The Future of Direct Participation

If State opinio juris and academic discourse are a gauge, a truce of sorts seems to have settled over the question of direct participation. The Interpretive Guidance has, with perhaps the exceptions of Part IX and select Part VII assertions with respect to a so-called Continuous Combat Function, proved a popular touchstone for academic studies of the direct participation standard. Still, the question of direct participation in hostilities has been characterized as “unresolved” by the ICRC.[2]

Following Parks’s lead, it seems everyone now agrees on direct participation as the conduct-based standard for attack. Yet what constitutes direct participation remains a matter of perspective. That is, disagreement whether direct participation refers to a proximate or uninterrupted connection between a civilian’s conduct and death, injury, or destruction to an enemy or whether it simply involves a good faith evaluation that civilian conduct amounts to an important or significant part of enemy action has been left to simmer.

For what it is worth, in addition to adoption by influential military doctrine, the latter view appears to better explain prevailing views on conduct such as scouting or providing target intelligence. Widely agreed to divest civilians of protection, scouting can in no plain sense of the words amount to “direct participation” as expressed by the Interpretive Guidance. A host of intervening acts must follow reports of enemy locations to produce death, injury, or destruction. But on a perhaps deeper level, whether the highly determinate standard including constitutive elements of a threshold of harm, direct causation, and a belligerent nexus offered by the Interpretive Guidance appropriately and pragmatically frames the conduct-based standard of attack or whether a flexible, though highly indeterminate standard is better-suited to conditions of combat remains a matter of judgment.

Whatever the future direction of this critical aspect of the law of war, from Parks’s early and later work on the subject, it is clear that rigorous legal analysis, sensitive to the hard-won legal bargains struck by States and informed by a sound grounding in military operational truths is essential. The forces that gave rise to adoption of the conduct-based standard, the legal ambiguities that reflect the limits of State consensus, and the unparalleled dangers and challenges of combat unrelentingly showcased by Parks should continue to play a prominent role in refinements of the legal notion of direct participation.


Sean Watts is a Professor in the Department of Law at the United States Military Academy, Co-Director of the the Lieber Institute for Law and Land Warfare at West Point, and Co-Editor-in-Chief of Articles of War.



[1] Department of the Army, Judge Advocate General’s Office of International Law, Memorandum of Law, “Law of War Status of Civilians Accompanying Military Forces in the Field,” May 6, 1999.

[2] Jakob Kellenberger, Foreword, Interpretive Guidance on the Notion of Direct Participation in Hostilities under International Humanitarian Law 6 (2009).