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?” ...