Henry Kissinger would rank in anyone’s top 10 list of the most important realists in the history of international relations theory and practice. As national security adviser and secretary of state, and as a prolific author, he became synonymous with dexterous, amoral diplomacy and a cold-blooded pragmatism attuned only to the balance of power and the pursuit of national interest. Robert Kaplan calls his “classical realism — as expressed in both his books and his statecraft — emotionally unsatisfying but analytically timeless.” Kissinger’s new book “World Order” reminds Walter Isaacson “why Realism matters.”

In fact, the most wondrous thing about “World Order” is that Kissinger, the uber-realist, has outed himself as a constructivist to the core. Forget about Kenneth Waltz’s spare realism of states jockeying for power and advantage under the relentless demands of anarchy. Enough with hard-boiled notions about the irrelevance of history, culture and identity or the scientific measurement of an objective balance of power. Nope. Kissinger’s reading of world order is that of Alexander Wendt, not Kenneth Waltz. We really are all constructivists now.

There has always been an unacknowledged strand of constructivism running through Kissinger’s academic writing, if not his statesmanship. Kissinger’s account of Europe in “World Order” recalls his outstanding early book “A World Restored,” which celebrated the efforts of early 19th century European statesmen to reconstruct international order around shared principles of legitimacy following the French Revolution and the Napoleonic wars. Insights about legitimacy and order ran through his magisterial book “Diplomacy.” But “World Order” offers by far Kissinger’s most explicit and unabashed embrace of constructivism. A decade ago, the George Washington University’s Henry Nau could uncontroversially juxtapose constructivists against Kissinger as “a foremost practitioner of Realist theory” interpreting world politics “in terms of the positioning and balancing of rival powers.” Not anymore.

Kissinger’s description of U.S. foreign policy now sounds less like Hans Morgenthau than John Ikenberry’s “Liberal Leviathan”: “the community of nations that they aimed to uphold reflected an American consensus – an inexorably expanding cooperative order of states observing common rules and norms, embracing liberal economic systems, forswearing territorial conquest, respecting national sovereignty, and adopting participatory and democratic systems of government.” The problem, for Kissinger, is that “today this ‘rules-based’ system faces challenges” because “there is no shared definition of the system.” This absence of a normative consensus is a recipe for instability: “any system of world order, to be sustainable, must be accepted – not only by leaders, but also by citizens.” His question, ultimately, is this: “Can regions with different cultures, histories, and traditional theories of order vindicate the legitimacy of any common system?”

That’s the sort of question that could have served as an epitaph for Alexander Wendt’s foundational work of constructivist international relations theory, “Social Theory of International Politics.” Wendt’s outline of a systemic constructivist alternative to realism defined multiple possible international orders in terms of precisely such terms. For Wendt, foreign policies are shaped profoundly and inextricably by the type of international order within which states existed: Hobbesian orders, anarchic and militaristic; Lockean orders, rule-governed and predictable; or Kantian orders, closely integrated by shared norms and democratic institutions. States internalized these norms to different degrees, and related to others not only through the balance of power but also in social terms such as “Friend” and “Enemy.” Wendt and a generation of constructivists showed that realist assumptions about the iron logic of anarchy and the security dilemma represented only one possible type of international order.

Like Wendt, Kissinger’s “World Order” evaluates regional and world politics in terms of degrees of legitimacy and the extent of a shared vision of international order. He identifies threats less by their material power than through their acceptance or rejection of prevailing principles of order. This emphasis leads him into an outsized and alarmingly undifferentiated portrayal of an Islamist challenge incorporating everyone from Iran to al-Qaeda to the Muslim Brotherhood. Realism, focused on material power, would care far less about such a threat, which manifests primarily through the realm of ideas and outside the framework of states.

Kissinger hasn’t forgotten about power, of course. Shifts in the balance of power repeatedly disrupt international order, with rising powers challenging the rules and fading powers struggling to hold on. But his conception of the balance of power is now thoroughly shaped by constructivist considerations of norms, identities and culture. Whether power shifts prove disruptive now depends upon the existence of legitimate shared institutions and norms, and constructing such a shared vision of order is the way to overcome periods of conflict. For Waltz and the neo-realists, power is something objective, based upon material capabilities such as size, population, resources, technological prowess, wealth and all that might go into the development of military might. Not for Kissinger, anymore, who observes that “in theory, [the balance of power] was based on realities; hence every participant in it should see it alike. But each society’s perceptions are affected by its domestic structure, culture, and history.”

It is not only Wendt’s structural version of constructivism, which runs through “World Order,” though. Kissinger has also fully internalized the constructivist idea that identity deeply shapes the foreign policies of states. “For nations,” he argues, “history plays the role that character confers on human beings.” Realism traditionally views foreign policy as governed by the pursuit of national interest as defined by the distribution of power in the system. While they argue incessantly over how states assess power and threats, most realists should agree at least that calculations of power and survival matter more for shaping these foreign policy choices than do identity, culture or – certainly! – morality.

Not Kissinger! A constructivist disposition runs through Kissinger’s analysis of both historical and current issues, with national identity and history looming larger in his discussions of individual countries than do systemic pressures. Like the authors in Peter Katzenstein’s path breaking “Culture of National Security,” Kissinger now highlights the domestic characteristics of state actors as much as he does their place within the balance of power. The realist might view Iranian-Saudi relations as a balance of power game under anarchy, but for Kissinger it is “above all a religious struggle, already lasting a millennium, between two wings of Islam.” The realist might see the negotiations over Iran’s nuclear program as a classic instance of strategic bargaining, but for Kissinger the real issue is Iran’s projection of a radical alternative to Western concepts of international order. Russia, Japan, China, India – all of their policies are, in his account, profoundly shaped by their civilizational history and visions of world order.

Kissinger hasn’t exactly gone soft, of course: “World Order” contains no apologies for a history of deception, violations of international law or complicity in massive human rights abuses. He still sees the military balance of power as a crucial driving force, and advocates a subtle, amoral diplomacy designed to maximize the national interest. And his constructivism is thin, failing to take into account decades of research on key questions such as how norms change, how power operates in different domains or how identities really shape political behavior. But this book’s understanding of world order, culture, history and identity presents a fully articulated constructivist narrative, which would have raised few eyebrows if penned by Katzenstein or Wendt. Kissinger’s implicit embrace of constructivism might have been a thermonuclear detonation in the Great International Relations Theory Paradigm War of the 1980s and 1990s. It is a measure of constructivism’s victory that nobody to this point even seems to have noticed.