In 1928, the Kellogg-Briand Pact was inaugurated to great international fanfare. Its signatories, who included the United States and most of Europe, disavowed the use of war in resolving international disputes and promised instead to employ only peaceful means. The accord was hailed as a triumph in the evolution of international relations. The rest, as they say, is history.

No one can deny that much of the twentieth century belonged to the realists and neorealists, those chilly mandarins who place the national interest above ideological high-mindedness. The last century was replete with examples of leadership tossing aside principle in favor of practical: Molotov and Ribbentrop, Nixon and Mao, Sadat and Begin, to name a few. With the end of the Cold War, however, international relations headed into uncharted waters. The twenty-first century presents realists of all stripes with complex new challenges. States are weaker and the logic of nuclear security is shifting. For realism to stay as relevant in the twenty-first century as in the twentieth, it must update itself.

Nonstates and Small States

At no time since the Treaty of Westphalia have nonstate actors played such a disproportionate role in international relations. Since the conclusion of the Thirty Years War in 1648, foreign affairs have been dominated by states. The system was seen as anarchic because states were the highest level in the paradigm and operated without any higher super-authority. This absence of an “international 911” forced states to serve as the final guarantors of their own security and strategize accordingly. The consequences of this reality have shaped much of human history for the last 350 years.

Today, international terrorism, multinational corporations, Wikileaks and numerous other nonstate entities drive the agenda as often as the bluster and bombast of individual states. Yet the old models have shown themselves to be glacial in adapting.

For example, Mutually Assured Destruction, one of the Cold War’s most sacral totems, was an early casualty. Its value was potent when one’s adversaries were cold Soviet atheists. But the doctrine is less helpful when they are religious fundamentalists—who not only do not fear their own demise but also seek it out as a strategic end. In their dealings with Hamas and Palestinian suicide terrorism, Israel has long understood this phenomenon.

The point was driven home in the West by September 11, yet the Bush doctrine that followed evinced the clunky and cumbersome challenges of grafting twentieth-century thinking onto a uniquely twenty-first-century conundrum. By targeting terrorists and “those who harbor them,” the president kept the Westphalian state-to-state conflict model intact, but at a cost of dangerously broadening the acceptable criteria for violence. What we got was Iraq.

In his seminal work, A Theory of International Relations, Kenneth Waltz, perhaps the most influential realist thinker alive today, famously dismissed nonstate actors as irrelevant. States, he said, were the highest unit, and among states only the most powerful should be considered. The great influence of the book notwithstanding, Waltz’s claim ran into numerous complications from the beginning: minor players like Cuba, North Korea and Vietnam have at times seemed to hold the fate of the world in their grasp, if not directly, then in their ability to induce major powers towards conflict. In the post-9/11 world, however, this element of Waltzian neorealism almost completely breaks down, as today even small states appear increasingly subject to the fissiparous dispersion of power.

Nuclear Weapons and Nuclear Peace

As a tool of international relations, few things have been as vexing as the dilemma of nuclear weapons. Unmatched in destructive power, the bomb has proliferated during one of the longest eras of global peace in human history. Since Hiroshima and Nagasaki, humanity has avoided the conflagrations that engulfed the world twice in the first half of the twentieth century.

Even the most ardent supporters of denuclearization today would be hard-pressed to deny at least some agency to nuclear weapons in preserving the peace of the world. The true test for realism and its competitors is how to apply this Cold War insight to a post–Cold War world.

Oxymoronic as it may sound, the idea of a “nuclear peace” does exhibit a graceful logic. If nuclear weapons kept the United States and the Soviet Union from blowing the world to bits out of nothing more than stone-cold fear, why then can such reasoning not be stretched to other nations, or even all nations—an ultimate Sword of Damocles? As Waltz articulated in a major 1981 paper, miscalculation and accidents would be unlikely as even the most foolish and irrational of actors could not help but be cognizant of the potential devastation conflict could bring.

The primary argument against adopting the theory as policy is that it would demand a level of accuracy that only hard science could provide. International relations is not physics. Even in recent years, as the field has sought to cloak itself in quantitative obscurantism, it remains a soft science. The profession’s luminaries are many, but there are no Newtons or Einsteins because international relations will always be limited to theory. There are no laws. The vagaries of human behavior will never conform to expectations as elegantly as an apple falling from a tree.

Nuclear peace demands 100 percent accuracy, but history already has shown this to be an impossibly high bar. In 1999, India and Pakistan fought over Kashmir in the Kargil War. Nuclear weapons went unused as cooler heads prevailed, but this will not always be the case if nuclear weapons continue to spread. Even if proliferation led the world to avoid nine hundred and ninety-nine of one thousand conflicts, that one exception might alone be worse than the sum of all we were spared.

The Importance of Theory

As the globe reverts back to multipolarity, circumstances such as nuclear weapons and the rise of nonstate forces threaten to make old forms of neorealism obsolete. This is not to say that the shortcomings of modern realism will herald the rise of a new neoliberal utopia. As Naazneen Barma, Ely Ratner and Steven Weber pointed out in the most recent issue of The National Interest, the liberal order and the renaissance of global governance that were supposed to follow the end of the Cold War remain mostly aspirational. While the influence of nonstate actors in the form of international organizations has increased, their ability to promote peace and transcend the dilemmas of anarchy has not. Academics have spent an inordinate amount of time attacking ideological opponents or reanalyzing the implications of the Punic Wars; as a result, they have allowed theory to atrophy.

This neglect of theory is dangerous. While it will never rise to the realm of law, theory remains critically important in providing signposts that guide U.S. foreign policy. For example, whether you agree with it or not, one of liberalism’s most lasting contributions has been the Democratic Peace Theory. This idea that mature democratic countries do not go to war with each other has been extraordinarily influential in guiding U.S. democracy-promotion efforts for the last twenty years. Realism today does not offer anything as compelling.

The coming decades promise a blizzard of international uncertainty. If the United States is to retain its eminence in the global order, it is essential that the marketplace of ideas in international relations remain vibrant and relevant. Realists need to do their part.

Jonathan Levine is a lecturer of American Studies and English at Tsinghua University in Beijing and a contributing analyst at Wikistrat, a geostrategic consulting group. You can follow him on Twitter at @LevineJonathan.