The claim that the liberal order produced the last seven decades of peace overlooks a major fact: the first four of those decades were defined not by a liberal order but by a cold war between two polar opposites. As the historian who named this “long peace” has explained, the international system that prevented great-power war during that time was the unintended consequence of the struggle between the Soviet Union and the United States. In John Lewis Gaddis’ words, “Without anyone’s having designed it, and without any attempt whatever to consider the requirements of justice, the nations of the postwar era lucked into a system of international relations that, because it has been based upon realities of power, has served the cause of order—if not justice—better than one might have expected.” 

During the Cold War, both superpowers enlisted allies and clients around the globe, creating what came to be known as a bipolar world. Within each alliance or bloc, order was enforced by the superpower (as Hungarians and Czechs discovered when they tried to defect in 1956 and 1968, respectively, and as the British and French learned when they defied U.S. wishes in 1956, during the Suez crisis). Order emerged from a balance of power, which allowed the two superpowers to develop the constraints that preserved what U.S. President John F. Kennedy called, in the aftermath of the Cuban missile crisis of 1962, the “precarious status quo.”

What moved a country that had for almost two centuries assiduously avoided entangling military alliances, refused to maintain a large standing military during peacetime, left international economics to others, and rejected the League of Nations to use its soldiers, diplomats, and money to reshape half the world? In a word, fear. The strategists revered by modern U.S. scholars as “the wise men” believed that the Soviet Union posed a greater threat to the United States than Nazism had. As the diplomat George Kennan wrotein his legendary “Long Telegram,” the Soviet Union was “a political force committed fanatically to the belief that with US there can be no permanent modus vivendi.” Soviet Communists, Kennan wrote, believed it was necessary that “our society be disrupted, our traditional way of life be destroyed, the international authority of our state be broken, if Soviet power [was] to be secure.” 

Before the nuclear age, such a threat would have required a hot war as intense as the one the United States and its allies had just fought against Nazi Germany. But after the Soviet Union tested its first atomic bomb, in 1949, American statesmen began wrestling with the thought that total war as they had known it was becoming obsolete. In the greatest leap of strategic imagination in the history of U.S. foreign policy, they developed a strategy for a form of combat never previously seen, the conduct of war by every means short of physical conflict between the principal combatants. 

To prevent a cold conflict from turning hot, they accepted—for the time being—many otherwise unacceptable facts, such as the Soviet domination of Eastern Europe. They modulated their competition with mutual constraints that included three noes: no use of nuclear weapons, no overt killing of each other’s soldiers, and no military intervention in the other’s recognized sphere of influence….

As is now clear, the end of the Cold War produced a unipolar moment, not a unipolar era. Today, foreign policy elites have woken up to the meteoric rise of an authoritarian China, which now rivals or even surpasses the United States in many domains, and the resurgence of an assertive, illiberal Russian nuclear superpower, which is willing to use its military to change both borders in Europe and the balance of power in the Middle East. More slowly and more painfully, they are discovering that the United States’ share of global power has shrunk. When measured by the yardstick of purchasing power parity, the U.S. economy, which accounted for half of the world’s GDP after World War II, had fallen to less than a quarter of global GDP by the end of the Cold War and stands at just one-seventh today. For a nation whose core strategy has been to overwhelm challenges with resources, this decline calls into question the terms of U.S. leadership.

This rude awakening to the return of history jumps out in the Trump administration’s National Security Strategy and National Defense Strategy, released at the end of last year and the beginning of this year, respectively. The NDS notes that in the unipolar decades, “the United States has enjoyed uncontested or dominant superiority in every operating domain.” As a consequence, “we could generally deploy our forces when we wanted, assemble them where we wanted, and operate how we wanted.” But today, as the NSS observes, China and Russia “are fielding military capabilities designed to deny America access in times of crisis and to contest our ability to operate freely.” Revisionist powers, it concludes, are “trying to change the international order in their favor.” ...