The complexity of the twenty-first century threat landscape contrasts markedly with the bilateral nuclear bargaining context envisioned by classical deterrence theory. Nuclear and conventional arsenals continue to develop alongside anti-satellite programs, autonomous robotics or drones, cyber operations, biotechnology, and other innovations barely imagined in the early nuclear age. The concept of cross-domain deterrence (CDD) emerged near the end of the George W. Bush administration as policymakers and commanders confronted emerging threats to vital military systems in space and cyberspace. The Pentagon now recognizes five operational environments or so-called domains (land, sea, air, space, and cyberspace), and CDD poses serious problems in practice. In Cross-Domain Deterrence, Erik Gartzke and Jon R. Lindsay assess the theoretical relevance of CDD for the field of International Relations. As a general concept, CDD posits that how actors choose to deter affects the quality of the deterrence they achieve. Contributors to this volume include senior and junior scholars and national security practitioners. Their chapters probe the analytical utility of CDD by examining how differences across, and combinations of, different military and non-military instruments can affect choices and outcomes in coercive policy in historical and contemporary cases.

Jon R. Lindsay is Assistant Professor at the Munk School of Global Affairs and Public Policy and the Department of Political Science at the University of Toronto. He previously served in the U.S. Navy with operational assignments in Europe, Latin America, and the Middle East. He is a co-editor of China and Cybersecurity (with Tai Ming Cheung, Derek S. Reveron, Oxford).

Erik Gartzke is Professor of Political Science and Director of the Center for Peace and Security Studies at the University of California, San Diego, where he has been a member of the research faculty since 2007. He is a co-editor of Nonproliferation Policy and Nuclear Posture (with Neil Narang and Matthew Kroenig) and Causes and Consequences of Nuclear Proliferation (with Robert Rauchhaus and Matthew Kroenig).

Introduction
1. Cross-Domain Deterrence, from Practice to Theory
Jon Lindsay and Erik Gartzke

The Concept of Cross-Domain Deterrence
2. Cross-Domain Deterrence in American Foreign Policy
Michael Nacht, Patricia Schuster, and Eva Uribe

3. The Past and Future of Deterrence Theory
Patrick M. Morgan

4. Simplicity and Complexity in the Nth Nuclear Era
Ron Lehman

Strategic Implications of Different Military Domains
5. Deterrence in and through Cyberspace
Jacquelyn Schneider

6. Anti-Satellite Weapons and the Instability of Deterrence
Benjamin Bahney, Jonathan Pearl, and Michael Markey

7. Air Power Versus Ground Forces: Deterrence at the Operational Level of War
Phil Haun

8. Sea Power Versus Land Power: Cross-Domain Deterrence in the Peloponnesian War
Joshua Rovner

Communication and Credibility across Domains
9. International Law and the Common Knowledge Requirements of Cross-Domain Deterrence
James Morrow

10. Signaling with Secrets: Evidence on Soviet Perceptions and Counterforce Developments in the Late Cold War
Brendan Rittenhouse Green and Austin G. Long

11. Extended Deterrence and Assurance in Multiple Domains
Rupal Mehta

Interactions across Military and Nonmilitary Domains
12. Asymmetric Advantage: Weaponizing People as Non-Military Instruments of Cross-Domain Coercion
Kelly Greenhill

13. Linkage Politics: Managing the End of the Cold War
Joshua R. Itzkowitz Shifrinson

14. Beyond Military Deterrence: The Multidimensionality of International Relations in East Asia
Chin-Hao Huang and David Kang

Conclusion
15. The Analytic Potential of Cross-Domain Deterrence
Jon Lindsay and Erik Gartzke