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Deterring gray zone aggression is more difficult than deterring interstate aggression is

• The authors identified eight core characteristics that determine when an activity qualifies as gray zone aggression. Although not all characteristics will always be true in a given case, gray zone aggression typically falls below the threshold for military response, unfolds gradually, is not attributable, uses legal and political justifications, threatens only secondary national interests, has state sponsorship, uses mostly nonmilitary tools, and exploits weaknesses and vulnerabilities in targeted countries and societies.

Deterrence is in a reasonably strong, though mixed, condition in each of the three examined contexts

• The U.S. and Japanese deterrence posture as it relates to Chinese aggression against the Senkaku Islands is in good shape. The authors rated all eight criteria for effective deterrence as either mixed or strong. For example, China’s motivation to use force to take the islands appears to be very low, and the United States and Japan have the ability to generate significant regional and global support for their response options.
• Regarding Russia’s gray zone aggression against the Baltic states, the U.S. and North Atlantic Treaty Organization deterrence posture is mixed. Only two of the eight criteria were judged to be strong, and two were judged to be weak. In particular, the United States and its local partners have not clearly communicated which outcomes they will not accept, making it unclear what they are trying to deter.
• The U.S. and South Korean deterrence posture as it relates to North Korean aggression against the South is also mixed, but the authors did not judge any of the criteria for effectiveness to be weak. One of the most important elements of this posture is ensuring and maintaining the close alignment between the United States and South Korea.

In an era of rising global competition, U.S. challengers and rivals are increasingly looking to achieve competitive advantage through gray zone activities — that is, acts of aggression that remain below the threshold of outright warfare. In this report, RAND researchers identify eight common characteristics of such aggression (e.g., unfolds gradually, is not attributable) and develop a framework for assessing the health of U.S. and partner deterrence in the gray zone. They apply the framework to three cases: China's aggression against the Senkaku Islands, Russia's aggression against the Baltic states, and North Korea's aggression against South Korea. The authors conclude that U.S. and partner deterrence of gray zone activities is in a reasonably strong, though mixed, condition in each of these three contexts. Finally, the authors outline the implications of their findings for the U.S. Army. Among these implications are that maintaining a local presence and posture plays an important role in conveying likely responses to aggression, and clear statements of shared intent to respond to specific actions are critical.

• Chapter One

  A Framework for Evaluating Gray Zone Deterrence

• Chapter Two

  Deterring China's Gray Zone Aggression Against the Senkaku Islands

• Chapter Three

  Deterring Russia's Gray Zone Aggression Against the Baltic States

• Chapter Four

  Deterring North Korea's Gray Zone Aggression Against South Korea

• Chapter Five

  Conclusions and Implications for the U.S. Army

• Appendix

  South Korea's Policy of Disproportionate Retaliation