Elizabeth K. Kiessling[*]
[Full text of this Article in PDF is available at this link]
States increasingly use their military forces to execute “gray zone tactics” in pursuit of strategic objectives. These tactics exceed the limits of accepted peacetime competition between states but avoid rising to a level that would warrant a conventional military response. Whether by design, necessity, or chance, these tactics fall somewhere between war and peace on the use-of-force spectrum, though exactly where they fall is difficult to say with any certainty. Do military-on-military gray zone tactics violate the prohibition on the threat of or use of force? Are gray zone tactics armed attacks? Something else entirely? The fact that gray zone tactics elude familiar categories of military action makes understanding potential responses difficult, as the range of permissible responses depends in part on how international law categorizes the initial act.
Where gray zone tactics involve one state using its military forces against another state, a use-of-force analysis is an appropriate starting point in evaluating these tactics and determining lawful response options. Yet, as this Article explains, many victim states may find the use-of-force framework unsatisfying. An alternative—and novel—approach is to evaluate whether such tactics violate the principle of non-intervention, which prohibits coercive actions intended to cause a victim state to do or refrain from doing something falling solely within the victim state’s domestic affairs.
Certain gray zone tactics clearly violate the principle of non-intervention. However, fitting military-on-military gray zone tactics into an unlawful intervention framework is likely to prove a difficult task in many instances, because these tactics often implicate international, as opposed to solely domestic, affairs. Given this reality and the frustrations that accompany a use-of-force analysis, victim states will likely continue to find current understandings of gray zone tactics and response options unsatisfying.
This Article explores the application and value of analyzing gray zone tactics through a non-intervention framework. Section I undertakes a deeper examination of the problem, including questions on the definition of, motivation behind, and effects of these tactics. Section II introduces three examples of military-on-military gray zone tactics: Russia’s use of military aircraft to fly close to U.S. warships; China’s seizure of a U.S. unmanned underwater vehicle; and Iran’s use of armed small boats to harass U.K. and U.S. warships. Section III briefly analyzes these examples under a use-of-force framework, illustrating the potential difficulties that may arise when approaching gray zone tactics from that paradigm. Sections IV and V then analyze these examples under the principle of non-intervention, which has been described as “one of the vaguest branches of international law.”
Considering levels of diplomatic and military tension among the major actors in current gray zone campaigns, how states understand and respond to gray zone tactics carries important consequences. If victim states are consistently unable to respond effectively, similar gray zone tactics may become normalized, thus permitting this type of state-to-state interaction. On the other hand, a mischaracterization of or hasty response to such tactics could result in the serious and rapid escalation of military engagements. Further, some states could conclude that current legal frameworks do not allow for effective responses to gray zone tactics. Those states may stretch their use-of-force determinations to encompass these tactics in order to expand response options—including those involving uses of force in self-defense—and such expanded responses could become the norm. Given these potential impacts on international norms, the legal challenges surrounding gray zone tactics must be clarified.
[*] Lieutenant Commander, Judge Advocate General’s Corps, U.S. Navy. LL.M., 2020, Harvard Law School; M.A., 2015, U.S. Naval War College; J.D., 2010, McGeorge School of Law; B.A., 2007, Gardner-Webb University. The views expressed herein are those of the author in her personal capacity and are not necessarily those of the U.S. Navy, the Department of Defense, or the U.S. government. The author especially wishes to thank Naz Modirzadeh for her encouragement and endless patience, Capt. Tom Leary for enduring countless “What if?” hypotheticals with characteristic good humor, and Rear Adm. (ret.) Gary Sharp for his valuable insights. Thanks also to Dustin Lewis, Cdr. Lena Whitehead, and the Harvard National Security Journal editorial staff, especially Kat Barragan, Patrick Byxbee, Alex Chemerkinsky, Christina Coleburn, Jon DeWitt, Rachael Hanna, Matthew Kahn, Annie Kapnick, Jillian Quigley, Sam Rebo, Ben Salvatore, Mtume Sangiewa, Noah Sissoko for their helpful comments and suggestions. Any errors are the author’s.
 While this Article focuses on actions taken by one State’s military against another State’s military, the use of gray zone tactics is not limited to military actions alone, nor are such tactics exclusive to State actors. For example, gray zone tactics could include economic or cyber actions by one State in an effort to influence foreign policy in another State; non-State armed groups and private corporations could also employ gray zone tactics. However, such tactics fall outside the scope of this Article.
 See Michael J. Mazarr, Mastering the Gray Zone: Understanding a Changing Era of Conflict 1 (2015); Lyle J. Morris, Michael J. Mazarr, Jeffrey W. Hornung, Stephanie Pezard, Anika Binnendijk & Marta Kepe, Gaining Competitive Advantage in the Gray Zone: Response Options for Coercive Aggression Below the Threshold of Major War 8 (2019); Sugio Takahashi, Development of Gray-zone Deterrence: Concept Building and Lessons From Japan’s Experience, 31 Pac. R. 787, 787–88 (2018); see generally James J. Wirtz, Life in the “Gray Zone:” observations for contemporary strategists, 33 Def. & Sec. Analysis 107 (2017); Aurel Sari, Legal Resilience in an Era of Grey Zone Conflicts and Hybrid Threats, 1–29 (ECIL Working Paper 2019/1), https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3315682 [https://perma.cc/Q2EE-8GVD].
 For one authoritative explanation of the non-intervention principle, see generally Maziar Jamnejad & Michael Wood, The Principle of Non-Intervention, 22 Leiden J. Int’l L. 345 (2009).
 P.H. Winfield, The History of Intervention in International Law, 1922 Brit. Y.B. Int’l L. 130, 130.