The subtitle tells the story: This original and provocative book contends that American retreat from global responsibilities and the “deprioritization” of alliances has emboldened revisionist powers—China, Iran, Russia—to probe at the outer limits of American power and influence. “We are at a dangerous moment in global geopolitics” (p. 13), the authors warn; “the American alliance system is in a state of advanced crisis” (p. 12).

To address the crisis they depict, the authors set out to “make the strategic case for America’s frontier alliances: why they matter, how we are losing them, and what America needs to do to preserve them for a new era” (p. 13). Which alliances do they have in mind? Drawing on Halford J. Mackinder’s concept of the “World Island” (“The Geographical Pivot of History,” Geographical Journal Vol. 170, No. 4, April 1904, 298–321) and Nicholas J. Spykman’s of its “Rimlands” (The Geography of the Peace, 1944), the authors delineate “a narrow belt that runs from the Baltic Sea to the Black Sea in Europe, through the Levant and Persian Gulf to the Indian Ocean and up through littoral Asia to the Sea of Japan” (p. 163). Why these regions in particular? They invoke Mackinder (Democratic Ideals and Reality, 1919, pp. 163–64): “Who rules East Europe commands the Heartland; who rules the Heartland commands the World-Island; who rules the World-Island commands the world.”

Invoking Mackinder and Spykman in an era of rapid globalization will strike some as outdated at best, but Jakub J. Grygiel and A. Wess Mitchell are in the company of Zbigniew Brzezinski (The Grand Chessboard, 1997, and The Choice, 2009) in this basic worldview, and they embrace some perspectives of geostrategists like George Kennan and Henry Kissinger. In an era of excessive fixation on globalization and interdependence, it is useful to be reminded that great power rivalries, alliances, and geography matter. Revisiting Mackinder and especially Spykman is a welcome contribution to the strategic debate, so long as they are lifted out of historical contexts that are less relevant for today’s challenges.

The strongest parts of the book are Chapters 3 and 4, “Revisionist Powers’ Probing Behavior” and “Responses of U.S. Allies” to these probes, which are insightful stand-alone pieces, independent of the book’s larger argument. Chapter 3 is a particularly useful guide for understanding the behavior of Vladimir Putin. Probes, in the authors’ conception, are meant to test a rival’s resolve without a direct confrontation (p. 48). They are low intensity and low risk, but they offer potentially high rewards. And such probes put the established power—the United States, in this case—in a bind, because they test its strength and resolve at the outer boundaries of its power and influence. Thus, Putin can be aggressive and cautious at the same time: He can probe using indirection and proxies, and he can retreat if confronted, waiting for another day to probe again when conditions are more propitious.

Chapter 4 offers a useful comparative analysis of allies’ “coping strategies,” ranging from accommodation to overly assertive military “self-help” (p. 112). One need not accept the authors’ argument that the United States has neglected its allies or been overly conciliatory toward “predatory” rivals to welcome this defense of the American alliance system as a vital element of global security and stability.

The question, then, is how the United States should counter such probing behavior. One obvious answer, which the authors reject, is to avoid being tested in areas where the revisionist power has all the advantages. For example, they assert (p. 64) that American credibility was damaged because Ukraine was not supported against Russian aggression starting in 2014 despite the security “assurances” given it in NATO’s Budapest Memorandum of 1994. But many—including most NATO allies—would argue that the mistake was to have given those assurances in the first place, incurring implied security responsibilities that would be very difficult to fulfill on Russia’s doorstep. Overextending U.S. security commitments or “assurances” does not strengthen but actually weakens the U.S. alliance system—as indeed has happened in NATO.

NATO’s initial eastward enlargement brought in Poland, Czechoslovakia, and Hungary, three mature countries in the heart of Europe, contiguous to the West, and with considerable military capacity. Their entry into the alliance in 1999 was a substantial plus for transatlantic security and the realization of a more united Europe. The next wave of enlargement was very different. It brought in, at U.S. insistence and in one fell swoop, 10 new members, many of them weak, vulnerable, and indefensible (except at prohibitive cost). They were brought in not as a matter of a sober security calculation, mindful that even American power has limits, but under the banner of “democratic enlargement,” as if NATO were some sort of democratic club, and as if security could be conferred with the stroke of a pen.

The authors nonetheless contend that NATO enlargement “removed the sources of conflict that had made this region the birthplace of three global wars in the twentieth century” (p. 124), and they would like the United States to take on more security commitments in the future. Indeed, they argue that the allies that should be given highest priority are precisely those that are weakest and most vulnerable, at the outer edge of America’s reach. They note that “alliances with small, vulnerable states are seen as offering a particularly low return on America’s strategic investment because of the size, remoteness and presumed indefensibility of these states” (p. 17). The authors vigorously dispute this view. It is precisely those states that we should focus on, they contend, because they are most vulnerable to the probing behavior of rival states. They cite Arnold Wolfers’s test of alliances—“whether the allies can offer something worthwhile in terms of military assistance” (p. 31)—but they turn this logic upside down to conclude that the most valuable allies are not the most capable and secure ones but those that are weak, vulnerable, and exposed. They conclude: “If there is an overriding priority for U.S. foreign policy, it is to restore America’s credibility among these states” (p. 165).

The argument must sound indefensible even to the authors, because they elsewhere speak of Israel, Poland, and Taiwan as being at the “outer limits of U.S. power” (p. 79). One thinks more of the Baltic states, Central Asia, the smaller Gulf states, and Southeast Asia as being at the “outer limits.” Israel, Poland, and Taiwan are capable, strong middle powers with active and robust security alliances with the United States.

How to provide security to “frontier allies”? The authors conclude that “the United States can no longer assume that large, multilateral defense alliances [referring to NATO specifically] will remain effective vehicles for reassuring frontline states” (p. 166). Instead, they propose to rely on “the main states in each region that possess the greatest incentive and ability to resist revisionist powers”—Poland, Saudi Arabia, Japan, and South Korea (p. 167). These prescriptions raise major questions. How could Poland take the lead in regional security if NATO writ large were as unreliable as the authors believe it to be? How could Saudi Arabia, as a prime incubator of jihadist ideology, play such a regional role, and why would we want it to? How could Japan and South Korea join forces to forge “a new regional security framework” without (and against) China, and why should they want to do so?

Oddly, the argument is not really about alliances at all, except in the narrowest sense of the term. Grygiel and Mitchell spend little time on the broad benefits conveyed by America’s postwar alliances in Europe and Asia; they spend no time at all considering how to reinvigorate NATO or the United States—European Union relationship to address twenty-first-century challenges. In Chapter 5, on “The Benefits of Alliances,” only six pages are devoted to their “geo-economic” benefit, and nothing at all is said of their contributions to counterterrorism, cybersecurity, environmental remediation, energy security, counterproliferation, humanitarian and development aid, and so on—or, for that matter, the imperative of engaging rivals on these same issues.

Rather, The Unquiet Frontier is an argument for what might be called the “Triple Containment” of Russia, China, and Iran. It describes a world divided between allies and predatory rivals that need to have their expansionist drives checked under a revitalized “Pax Americana” (p. 123). The authors acknowledge that “America retains many hegemonic capabilities and characteristics—including the forward deployed system of alliances and security commitments that America continues to maintain in their own neighborhoods—that present real obstacles to aspirant powers” (p. 9), but they fail to consider that this intrusion into the neighborhoods of other countries might be seen as threatening to them and might give rise to the very behavior that the authors deem “predatory.” It is the familiar “security dilemma” in international affairs, whereby the actions one state takes to be defensive are seen by another state as offensive and aggressive, requiring countermeasures on its part.

One of the problems with containment is that it is reactive, allowing the other side to dictate the time and place of battle on terms most advantageous to itself. Because it is threat based, it can cause countries to lose sight of their own core values, interests, and objectives, which can be held hostage to the overarching imperative of countering the aspirations (or probes) of revisionist states, whether enduring interests are at stake or not. It is, moreover, a strategy more appropriate to the bipolar world of the Cold War than to one in which alliances are inevitably less cohesive and like-minded (as the authors note) and in which rival powers often have interests that overlap, even if imperfectly, with those of the established powers. The task ahead is to adapt and modernize our existing bilateral and multilateral alliances, while also calibrating our policies to take account of the legitimate interests of other major powers, even revisionist ones—and to engage those powers where we can in addressing such challenges as nuclear proliferation and international terrorism.