In this valuable new book, Austin Carson examines the phenomenon of covert military intervention, which he defines as occurring when an external major power secretly provides military assistance during war. Carson argues that such interventions are more common than might be expected and that they often lead to a puzzling dynamic, whereby an adversary detects the intervention but does not publicize it.

Carson’s use of the term “covert” follows the conventional definition of government activities designed to conceal the actor’s role in that activity, but by focusing on covert military intervention, he is studying a phenomenon different from “covert action,” which in the American context usually refers to activities undertaken by the Central Intelligence Agency, rather than by military forces. This book is therefore a complement to other works on covert action, including Lindsey A. O’Rourke’s recent Covert Regime Change: America’s Secret Cold War.

The book provides several insights for students of international relations, such as that covert intervention was a more important tool for the major powers during the Cold War than has been widely recognized. Especially fascinating is Carson’s discussion of what he calls “open secrets”: supposedly covert interventions that become known not only to the adversary but to the public through exposure by the media or other actors. And yet the government conducting the intervention continues to maintain the veil of secrecy. Carson cites examples of such “exposed but officially unacknowledged” activities, including Israel’s nuclear weapons program, America’s drone strike program in Pakistan, and Russia’s use of “little green men” in Eastern Ukraine (p. 10). We could add other prominent examples of this phenomenon, such as the widely reported but unacknowledged joint U.S. and Israeli cyberattacks against Iranian nuclear facilities known as Operation Olympic Games.

Carson builds his theory on several simplifying assumptions that seem a little questionable, such as that covert interventions will always be detected by the other side and that leaders making decisions about covert interventions will face hawkish (rather than dovish) pressures from domestic audiences. But his argument seems plausible: that a shared desire to limit war will encourage potential interveners to do so covertly while also motivating rival leaders to refrain from publicizing the fact of the other side’s intervention once they find out about it. And even if the intervention becomes widely known—an open secret—leaders will use continued non-acknowledgment to insulate themselves from domestic hawkish pressures that would otherwise push them to expand the intervention and possibly provoke a full-scale war.

Carson’s empirical data consists of case studies of five wars: the Spanish Civil War, Korean War, Vietnam War, Soviet occupation of Afghanistan, and U.S. occupation of Iraq. Each case except for the U.S. occupation of Iraq involves several subcases of interventions during that conflict, such as interventions during the Korean War by the United States, the Soviet Union, and China. The first four of these cases are each given a chapter, while the final case of the U.S. occupation of Iraq is covered more briefly in the concluding chapter. The cases provide fresh insight into relatively little known aspects of these conflicts, such as the covert air war between American and Soviet pilots during the Korean War.

The book is all the more significant today, as the threat of great-power competition is returning to the center of the world stage, and it seems likely that we will see more covert interventions in the future.