The absence of great-power wars since 1945 did not happen by accident. A large measure of grace and good fortune is part of the story. But the experience of catastrophic war also compelled the architects of the postwar order to attempt to bend the arc of history. American leaders’ personal experiences of winning the war gave them the confidence to think the unthinkable and to do what previous generations had dismissed as undoable by constructing an international order that could bring peace. To ensure that this long peace continues, American leaders and citizens alike need to recognize what an amazing achievement it has been, to realize how fragile it is, and to begin a serious debate about what will be required to sustain it for another generation.
A MIRACULOUS ACHIEVEMENT
Three numbers capture the defining features—and successes—of the international security order: 80, 80, and nine. It has been 80 years since the last hot war between great powers. This has enabled the global population to triple, life expectancy to double, and global GDP to grow 15-fold. If, instead, post–World War II statesmen had settled for history as usual, a third world war would have occurred. But it would have been fought with nuclear weapons. It could have been the war to end all wars.
It has also been 80 years since nuclear weapons were last used in war. The world has survived several close calls—most dangerously the Cuban missile crisis, when the United States faced off with the Soviet Union over nuclear-tipped missiles in Cuba and during which President John F. Kennedy estimated the odds of nuclear war to be between one in three and one in two. More recently, in the first year of Russia’s full-scale war on Ukraine, which began in 2022, Russian President Vladimir Putin seriously threatened to conduct tactical nuclear strikes. According to reporting in The New York Times, the CIA estimated the odds of a Russian nuclear strike to be 50-50 if Ukraine’s counteroffensive were about to overrun retreating Russian forces. In response, CIA director Bill Burns was dispatched to Moscow to convey American concerns. Fortunately, imaginative collaboration between the United States and China dissuaded Putin, but it served as a reminder of the fragility of the nuclear taboo—the unstated global norm that the use of nuclear weapons should be off the table.
In the 1950s and 1960s, world leaders expected countries to build nuclear weapons as they acquired the technical capability to do so. Kennedy predicted that there would be 25 to 30 nuclear-armed states by the 1970s, which led him to promote one of the boldest initiatives of American foreign policy. Today, 185 states have signed the Nuclear Nonproliferation Treaty forswearing nuclear weapons. Remarkably, only nine countries have nuclear arsenals.
Like the 80 years of peace and the absence of nuclear wars, the nonproliferation regime—of which the treaty has become the centerpiece—is also a tenuous achievement. More than 100 countries now have the economic and technical base to build nuclear weapons. Their choice to rely on the security guarantees of others is geostrategically and historically unnatural. Indeed, a 2025 Asan Institute poll found that three-quarters of South Koreans now favor acquiring their own nuclear arsenal to protect against North Korea’s threats. And if Putin is able to advance his war aims by ordering a tactical nuclear strike on Ukraine, other governments will likely conclude that they need their own nuclear shield.
THE END OF AN ERA
In 1987, the historian John Lewis Gaddis published a landmark essay titled “The Long Peace.” It had been 42 years since the end of World War II, an era of stability comparable to that between the Congress of Vienna in 1815 and the Franco-Prussian War in 1870, and the decades after that until the outbreak of World War I in 1914. Gaddis argued that the foundation of this modern long peace was the Cold War. In structural conditions that would in earlier eras almost certainly have led to a third world war, the United States and the Soviet Union confronted each other with arsenals sufficient to withstand a nuclear strike and retaliate decisively. Nuclear strategists described this as mutually assured destruction, or MAD.
In addition to the establishment of the United Nations, the UN Declaration of Human Rights, the multilateral arrangements that eventually developed into the European Union, and the fierce ideological dimension of the U.S.-Soviet rivalry, the central causal factor for peace, Gaddis argued, was the mutual judgment that systemic interests trumped ideological ones. The Soviets hated capitalism and Americans rejected communism. But their desire to prevent mutual destruction was more important. As he explained, “The moderation of ideologies must be considered, then, along with nuclear deterrence and reconnaissance, as a major self-regulating mechanism in post-war politics.”
As Gaddis recognized, the world had split into two camps in which each superpower sought to attract allies and aligned countries around the globe. The United States launched the Marshall Plan to rebuild Western Europe, established the International Monetary Fund and the World Bank to promote global development, and pushed for the General Agreement on Tariffs and Trade to set the rules of economic exchange to promote economic growth. The United States even abandoned its prior strategy of trying to avoid entangling alliances—an idea that dates back to George Washington’s presidency—by embracing the North Atlantic Treaty Organization and a treaty commitment to Japan. It pursued any option available to build an international security order that could counter the threat of Soviet communism. As one of us (Allison) explained in Foreign Affairs, “Had there been no Soviet threat, there would have been no Marshall Plan and no NATO.”
The foundation of the modern long peace was the Cold War.
After the fall of the Soviet Union, in the early 1990s, triumphalists hailed a new unipolar era in which only the United States remained as a great power. This new order would bring a peace dividend in which countries could flourish without worrying about great-power conflict. The dominant narratives of the first two decades after the Soviet Union’s collapse even declared “the end of history.” In the words of the political scientist Francis Fukuyama, the world was witnessing “the end point of mankind’s ideological evolution and the universalization of Western liberal democracy as the final form of human government.” Using the example of McDonald’s restaurants, Thomas Friedman’s “Golden Arches Theory of Conflict Prevention” argued that economic development and globalization would ensure an era of peace. These ideas informed the U.S. invasions of Afghanistan and Iraq, which left the United States bogged down in endless, winless wars for two decades.
Creative diplomacy was also an essential strand in this chapter of the story. The disintegration of the Soviet Union and the emergence of Russia and 14 newly independent Eastern European states should have meant a surge of nuclear-armed countries. More than 12,600 nuclear weapons were left outside Russia when the Soviet Union collapsed. It took an extraordinary partnership between the United States and Russian leader Boris Yeltsin’s democratizing Russia, funded by a cooperative denuclearization program spearheaded by U.S. Senators Sam Nunn and Richard Lugar, to ensure that these weapons did not fall into the wrong hands. By 1996, teams had removed every nuclear weapon from former Soviet territory and either returned them to Russia or dismantled them.
The geopolitical changes after the fall of the Soviet Union had reset U.S. relations with both its former adversaries and its growing challengers. In 2009, when Barack Obama was inaugurated as U.S. president, both Russia and China were characterized as “strategic partners.” This remained the dominant view. But by the time Donald Trump became U.S. president in 2017, the reality of an ambitious, rapidly rising China and a resentful, revanchist Russia led to the recognition that the United States had entered a new era of great-power competition.
DANGERS AHEAD
Before his death, in 2023, Henry Kissinger repeatedly reminded colleagues that he believed these eight decades of great-power peace were unlikely to reach a full century. Among the factors that history shows contribute to the violent end of a major geopolitical cycle, five stand out that could bring the ongoing long peace to a close.
At the top of the list is amnesia. Successive generations of American adults, including every serving military officer, have no personal memory of the horrible costs of a great-power war. Few people recognize that, before this exceptional era of peace, a war in each generation or two was the norm. Many today believe that a great-power war is inconceivable—failing to recognize that this is not a reflection of what is possible in the world but of the limits of what their minds can conceive.
The existence of rising competitors also threatens peace. China’s meteoric ascent is challenging U.S. preeminence, echoing the type of fierce rivalry between an established and a rising power that the ancient Greek historian Thucydides warned would lead to conflict. At the beginning of the twenty-first century, the United States did not give much thought to competing with China, which was far behind economically, militarily, and technologically. Now, China has caught up or even surpassed the United States in numerous areas, including trade, manufacturing, and green technologies, and is rapidly advancing in others. At the same time, Putin, who presides over a weakening country but still commands a nuclear arsenal capable of destroying the United States, has demonstrated his readiness to use war to restore a measure of Russian greatness. With Russian threats mounting and the Trump administration’s support for NATO declining, Europe is struggling to come to terms with acute security challenges in the decades ahead.
Global economic leveling further increases the possibility of war. American economic predominance has eroded as other countries have recovered from the devastation of the two world wars. At the end of World War II, when most other major economies had been destroyed, the United States had one-half of the world’s GDP; when the Cold War ended, the U.S. share had fallen to one-quarter. Today, the United States has only one-seventh. With this shift in the balance of national economic power, a multipolar world is emerging in which multiple independent states can act within their spheres of influence without asking permission or fearing punishment. This erosion accelerates when the dominant power overextends itself financially, as the celebrated hedge fund manager Ray Dalio argues the United States is doing today.
Successive generations of American adults have no personal memory of a great-power war.
When an established power also overextends itself militarily—especially in conflicts that rank low on a list of its vital interests—its ability to deter or defend against rising powers weakens. The ancient Chinese philosopher Sun-tzu wrote, “When the army engages in protracted conflicts the resources of the state will fall short,” which could describe the costly mission creep of U.S. forces in Iraq and Afghanistan and the military’s inability to focus on more pressing challenges. Narrowly concentrating resources on these extended conflicts drew the United States’ attention away from improving its defense capabilities against increasingly sophisticated and more dangerous adversaries. Of even greater concern is the extent to which the U.S. national security establishment has fallen into a vicious cycle, supported by Congress and the defense industry, in which it demands more means—increased funding—rather than looking for more strategic ways to address grave threats to its national interests.
Finally, and most concerning, the tendency of an established power to descend into bitter political divisions at home paralyzes its ability to act coherently on the world stage. This is particularly troublesome when leaders oscillate between opposing positions on whether and how the country should maintain a successful global order. This is unfolding today: an ostensibly well-meaning administration in Washington is upending nearly every existing international relationship, institution, and process to impose its view of how the international order must change.
Long-wave geopolitical cycles do not last forever. The most important question facing Americans and the divided U.S. polity is whether the nation can gather itself to recognize the perils of the moment, find the wisdom required to navigate it, and take collective action to prevent—or more accurately, postpone—the next global convulsion. Unfortunately, as Hegel observed, we learn from history that too often we do not learn from history. When American strategists crafted the Cold War strategy that was the foundation of the long peace, their vision lay far beyond the conventional wisdom of earlier eras. To sustain the exception that has allowed the world to experience an unprecedented period without a great-power war will require a similar surge of strategic imagination and national determination today.