In this article published in Foreign Affairs in late November 2025, Graham Allison and James A. Winnefeld Jr. argue that the world is living through a rare historical exception. Nearly eight decades without a direct war between major powers represents the longest such period since Roman times. Yet this exception is now under increasing strain.
The authors contend that the “long peace” after 1945 was not the result of coincidence. Rather, it was the product of deliberate strategic engineering that shaped the foundations of the international order after the Second World War. Preserving it today requires a comparable degree of strategic imagination and political will, lest the world slide into a logic of escalation that reproduces great power wars.
Graham Allison is a professor of governance at Harvard University and one of the most prominent theorists of national security and decision making. He previously served as the founding dean of the Kennedy School and authored landmark works on nuclear strategy and US relations with Russia and China.
James A. Winnefeld Jr. is a retired US Navy admiral who served as Vice Chairman of the Joint Chiefs of Staff, the second highest military position in the United States. Before that, he held senior operational and strategic leadership roles, giving the article additional weight at the intersection of academic analysis and practical experience in the management of force and the risks of war.
The past eight decades represent the longest period without war between great powers since the era of the Roman Empire. This exceptional phase of extended peace followed two catastrophic wars, each far more destructive than previous conflicts, to the extent that historians were compelled to invent an entirely new classification to describe them: world wars. Had the remainder of the twentieth century been marked by the same level of violence as the preceding thousand years, the lives of most people alive today would have been fundamentally different.
The absence of wars between major powers since 1945 was not purely accidental. While chance and good fortune certainly played a role, the experience of catastrophic war itself drove the architects of the postwar international order to attempt to divert history from its traditional course. The personal experience of American leaders in achieving victory in war gave them the confidence to think what had previously been considered impossible and to undertake what earlier generations deemed unattainable, by constructing an international system capable of sustaining peace.
The Historic Peace
Three numbers capture the decisive features and successes of the international security system: 80, 80, and 9. Eighty years have passed since the last hot war between major powers. This allowed the world’s population to triple, average life expectancy to double, and global gross domestic product to grow fifteenfold. Had post Second World War statesmen simply accepted what history had accustomed them to, a third world war would have erupted, likely fought with nuclear weapons, and perhaps would have been the war that truly ended all wars.
It has also been eighty years since nuclear weapons were last used in war. The world survived several dangerous moments when nuclear use came perilously close, the most severe being the Cuban Missile Crisis, when the United States confronted the Soviet Union over nuclear missiles deployed in Cuba. At that time, US President John Kennedy estimated the probability of nuclear war at between 33 percent and 50 percent.
More recently, in the first year of Russia’s full scale war against Ukraine that began in 2022, Russian President Vladimir Putin seriously threatened to carry out tactical nuclear strikes. According to reports by The New York Times, the US Central Intelligence Agency assessed the likelihood of a Russian nuclear strike at 50 percent if Ukrainian counteroffensives were on the verge of overrunning retreating Russian forces.
In response, CIA Director Bill Burns was dispatched to Moscow to convey US concerns. Fortunately, a novel form of cooperation between the United States and China succeeded in deterring Putin. This nonetheless served as a reminder of the fragility of the “nuclear red line”, the unwritten global norm that the use of nuclear weapons should not be an option for any state that possesses them.
In the 1950s and 1960s, world leaders expected states to pursue nuclear weapons as soon as they acquired the technical capacity to do so. Kennedy predicted that 25 to 30 countries would become nuclear armed by the 1970s. This spurred him to promote one of the boldest initiatives in the history of US foreign policy, the Nuclear Non Proliferation Treaty. To date, 185 states have signed it, committing to forgo nuclear weapons, while only nine states possess nuclear arsenals.
Just as eighty years of peace and the absence of nuclear war constitute an extraordinary achievement, the non proliferation regime, with the treaty as its cornerstone, is itself a fragile accomplishment. Today, more than one hundred states possess the economic and technical base required to build nuclear weapons. They have chosen instead to rely on security guarantees provided by other countries, a choice that is historically and geopolitically unusual.
A 2025 survey conducted by the Asan Institute found that 75 percent of South Koreans now support their country acquiring its own nuclear arsenal to protect itself from North Korean threats. If Putin were to achieve his wartime objectives through the use of a tactical nuclear strike against Ukraine, other governments would likely conclude that they too require their own nuclear umbrella.
The End of a Phase
In 1987, historian John Lewis Gaddis published a seminal article titled “The Long Peace”. At the time, forty two years had passed since the end of the Second World War, a period of stability comparable to the era between the Congress of Vienna in 1815 and the Franco Prussian War in 1870, and the decades that followed until the outbreak of the First World War in 1914.
Gaddis argued that the foundation of this modern long peace was the Cold War. Under structural conditions that in earlier eras would almost certainly have led to a third world war, the United States and the Soviet Union stood face to face, each possessing an arsenal sufficient to absorb a nuclear strike and retaliate decisively. Nuclear strategists described this condition as mutually assured destruction, known as MAD.
Alongside the establishment of the United Nations, the Universal Declaration of Human Rights, multilateral arrangements that later evolved into the European Union, and the sharp ideological dimension of US Soviet rivalry, Gaddis identified the decisive factor behind the long peace as the mutual judgement in Washington and Moscow that material and systemic interests outweighed ideological ones.
The Soviets despised capitalism and Americans rejected communism, yet each side’s desire to prevent mutual nuclear annihilation outweighed all else. As Gaddis explained, the taming of ideologies, alongside nuclear deterrence and surveillance, must be regarded as a principal mechanism of self restraint in postwar politics.
As Gaddis also observed, the world divided into two camps, with each great power seeking to attract allies and aligned states across 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 supported the General Agreement on Tariffs and Trade to set rules for economic exchange that fostered growth. The United States even abandoned its earlier strategy of avoiding alliances that entangled it in obligations, instead adopting the North Atlantic Treaty Organization and committing to a defence treaty with Japan.
Washington pursued every available option to build an international security system capable of confronting the Soviet communist threat. As Allison noted in a previous Foreign Affairs article, without the Soviet threat there would have been no Marshall Plan and no NATO.
After the collapse of the Soviet Union in the early 1990s, triumphalist theorists proclaimed a new unipolar era in which the United States remained the sole superpower. This order was expected to generate a “peace dividend”, allowing states to prosper without fear of great power conflict. Narratives in the first two decades after the Soviet collapse even declared the “end of history”.
In the words of political theorist 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 argued in the “Golden Arches Theory of Conflict Prevention” that economic development and globalization would guarantee an era of peace. These ideas contributed to the US invasions of Afghanistan and Iraq, which left the United States mired in endless wars with no victory for two decades.
Creative diplomacy was also a central thread in this chapter. The Soviet Union disintegrated and Russia emerged alongside fourteen new independent states in Eastern Europe. In theory, this could have meant a dramatic increase in the number of nuclear armed countries, as more than 12,600 nuclear warheads were left outside Russia at the moment of Soviet collapse.
An extraordinary partnership between the United States and Russia under Boris Yeltsin, combined with funding from a cooperative disarmament program led by US senators Sam Nunn and Richard Lugar, was required to ensure these weapons did not fall into undesirable hands. By 1996, all nuclear weapons had been removed from the territory of the former Soviet Union, either dismantled or returned to Russia.
Geopolitical transformations after the Soviet collapse redefined US relations with both former adversaries and rising competitors. In 2009, when Barack Obama assumed the US presidency, both Russia and China were described as “strategic partners”. This perception persisted until 2017, when Donald Trump became president. The reality of China’s rapid and ambitious rise, alongside a Russia resentful of the status quo, led Washington to recognize that the United States had entered a new era of great power competition.
Danger on the Horizon
Before his death in 2023, former US Secretary of State Henry Kissinger periodically reminded colleagues that he did not believe these eight decades of peace between major powers would reach a full century. Historical experience points to five specific factors that could ultimately bring the long peace to a violent end.
At the forefront is forgetting. Successive generations of American adults, including all active duty military officers today, have no personal memory of the horrific costs of a war between major powers. Few realize that before this exceptional era, war in every generation or two was the norm. Many today believe that a great power war is inconceivable, without understanding that this reflects not what is possible in the world, but only the limits of their own imagination.
The presence of rising challengers is the second factor threatening peace. China’s meteoric rise challenges US dominance, repeating the pattern of intense rivalry between an established power and a rising one. This pattern was famously identified by the ancient Greek historian Thucydides as a pathway to conflict. At the start of the twenty first century, the United States paid little attention to competition with China, which lagged economically, militarily, and technologically. Today, China has caught up with or surpassed the United States in many areas, including trade, manufacturing, and green technologies, and is advancing rapidly in others.
At the same time, Vladimir Putin, who rules a state growing weaker yet still possessing a nuclear arsenal capable of destroying the United States, has shown his willingness to use war to restore a measure of Russia’s former prestige. With rising Russian threats and declining support from the Trump administration for NATO, Europe finds itself struggling to confront severe security challenges in the decades ahead.
Global economic convergence also increases the likelihood of war. US economic supremacy eroded as other countries recovered from the devastation of the two world wars. At the end of the Second World War, when most other major economies lay in ruins, the United States held half of global GDP. By the end of the Cold War, its share had fallen to a quarter.
Today, the United States holds only one seventh. As national economic power balances shift, a multipolar world emerges in which many independent states can act within their spheres of influence without seeking permission or fearing punishment. This erosion accelerates when the dominant power overextends financially, as hedge fund manager Ray Dalio argues the United States is doing today.
When the established power also overextends militarily, especially in conflicts that rank low among its vital interests, its ability to deter or defend against rising powers weakens. The ancient Chinese philosopher Sun Tzu wrote, “When an army engages in prolonged campaigns, the resources of the state are exhausted.” This description applies to the costly expansion of US military missions in Iraq and Afghanistan over the past two decades, and to the inability of the US military establishment to focus on more urgent challenges.
The narrow concentration of resources on these protracted conflicts diverted US attention from improving its defensive capabilities against increasingly sophisticated and dangerous adversaries. What is most concerning now is the extent to which the US national security establishment is sliding into a vicious cycle, supported by Congress and defence industries, in which it demands more means, meaning funding, instead of seeking more strategic approaches to confronting grave threats to national interests.
Finally, the most dangerous factor is the tendency of the established power to slide into bitter internal political divisions that paralyze its ability to act coherently on the global stage. This problem deepens when leaders oscillate between opposing views on whether the country should sustain a successful global order and how to do so. This is evident today, as the administration in Washington, which appears well intentioned on the surface, works to overturn many existing international relationships, institutions, and processes in order to impose its own vision of how the international system should change.
Long term geopolitical cycles do not last forever. The most important question facing Americans, and a deeply divided US political community, is whether the nation is capable of pulling itself together to recognize the dangers of the present moment, find the wisdom needed to navigate it, and organize collective action that prevents, or at least delays, the coming global upheaval.
Unfortunately, as the German philosopher Hegel observed, what we learn from history is that humans often do not learn from history. When American strategists crafted the Cold War strategy that formed the basis of the long peace, their vision extended far beyond the prevailing wisdom of earlier eras. Preserving this exception, which allowed the world to experience an unprecedented period without war between major powers, will today require a similar surge of strategic imagination and national resolve.
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