The long peace of the past eight decades has rested on two revolutionary convictions: that wars of aggression are intolerable and that empires must end. The first principle emerged from the carnage of two world wars, which together killed a hundred million people. The second came from centuries of colonial subjugation and the fight across Asia, Africa, and Latin America for self-determination. The United Nations Charter, signed in San Francisco in June 1945, gave both convictions political form.

Since then, the world has avoided a cataclysmic great-power war. Even more remarkably, global European empires were dismantled and replaced by a new system of nearly 200 sovereign states. Both achievements combined to make possible extraordinary advances in human well-being. To be sure, the world has witnessed many conflicts since the end of World War II, including savage wars of decolonization, and soaring economic growth has come alongside deep inequalities and environmental destruction. But it remains indisputable that for billions of people, the past 80 years have been a time of peace and rising prosperity.

That era is now coming to an end. The twin convictions of no war and no empire, the load-bearing walls of the long peace, are fast buckling. The symptoms are clear. Interstate and civil wars have mushroomed in recent years, bringing immeasurable suffering to hundreds of millions. The great powers themselves have launched wars of aggression, including Russia’s invasion of Ukraine and the joint U.S.-Israeli war against Iran. Nuclear powers are modernizing and expanding their arsenals, strategic arms control agreements have lapsed, nuclear facilities have come under direct military attack in recent years, and states big and small are arming themselves at a pace unseen since the 1980s. In Ukraine, Palestine, Sudan, and now Iran, diplomacy appears only in its most anemic or ad hoc form. And as conflict spreads, United Nations peacemaking is missing in action.

The standard explanation for the unfolding crisis is the collapse of the so-called rules-based or liberal international order, a post–Cold War arrangement tied to American military and financial supremacy. Washington’s retreat from multilateral alliances and institutions such as NATO and the World Trade Organization means this order is indeed breaking down. But this analysis conflates two very different things. The liberal international order was not what produced the long peace. Instead, and in significant ways, it undermined the twin convictions on which the peace has actually rested.

The real disaster is the abandonment of the twin convictions of no war and no empire by states and publics alike, caused not by the American retreat but by an erosion of the international moral leadership and collective memory that once sustained them. It’s a crisis of imagination produced by a compound amnesia, not just of war and empire but also of the extraordinary peacemaking successes of an earlier United Nations. Recovering that lost history and rebuilding the politics (and only then, the institutions) that once placed the twin convictions at the center of global thinking are the essential first steps toward a new, peaceful global order.

This lost history matters all the more because the world that is emerging, in which no single power can organize international politics around its own preferences, resembles more the world of the earlier United Nations (from roughly 1955 to 1990) than that of the past three and a half decades of American supremacy. The recent U.S.-Israeli war against Iran may be a harbinger of future conflicts, an interstate war in which one or more sides will need what is now often termed an “off-ramp.” In the past, it was precisely when warring parties approached exhaustion or were wary of escalation but could find no ready exit that UN secretaries-general proved indispensable, time and again, crafting not just any off-ramp but one that safeguarded future peace by reinforcing the taboo against wars of aggression and the imperative that the age of empire must not return.

The United Nations can be rebuilt. What’s needed is less an institutional fix (however important reform in the membership of the Security Council, for example, may be) and more the restoration of the twin convictions: through political leadership willing to champion them, a new UN secretary-general who will demonstrate them in action, and a global public demanding once more a world with neither war nor empire. Past UN peacemaking was possible not because the institutions were perfect—they never were—but because the convictions animating those institutions were politically alive, defended and advanced by states and peoples determined to keep them at the heart of international relations.

The United Nations was not born as a liberal project. It was first conceived as a muscular continuation of the wartime alliance, a collective security mechanism that would crush future aggression, with U.S. and Soviet bomber planes operating jointly from air bases circling the globe. But other, more expansive visions of the UN imagined it as an organization that would include the voices of smaller states working together toward a better world. The charter represented a compromise. By the time of its signing 81 years ago, Washington and Moscow were already beginning to eye each other as adversaries and wanted to make sure that the UN would above all prevent........

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