The Cold War as a system of states ended on a cold and gray December day in Moscow in 1991, when Mikhail Gorbachev signed the Soviet Union out of existence. Communism itself, in its Marxist-Leninist form, had ceased to exist as a practical ideal for how to organize society.

“If I had to do it over again, I would not even be a Communist,” Bulgaria’s deposed Communist leader, Todor Zhivkov, had said the year before. “And if Lenin were alive today, he would say the same thing. I must now admit that we started from the wrong basis, from the wrong premise. The foundation of socialism was wrong. I believe that at its very conception the idea of socialism was stillborn.”

But the Cold War as an ideological struggle disappeared only in part, despite Communism’s implosion. On the American side, not so much changed on that day. The Cold War was over, and the United States had won it. But most Americans still believed that they could only be safe if the world looked more like their own country and if the world’s governments abided by the will of the United States.

Ideas and assumptions that had built up over generations persisted, despite the disappearance of the Soviet threat. Instead of a more limited and achievable American foreign policy, most policy makers from both parties believed that the United States could then, at minimal cost or risk, act on its own imperatives.

America’s post-Cold War triumphalism came in two versions. First was the Clinton version, which promoted a prosperity agenda of market values on a global scale. Its lack of purpose in international affairs was striking, but its domestic political instincts were probably right: Americans were tired of foreign entanglements and wanted to enjoy “the peace dividend.”

As a result, the 1990s was a lost opportunity for international cooperation, particularly to combat disease, poverty and inequality. The most glaring examples of these omissions were former Cold War battlefields like Afghanistan, Congo and Nicaragua, where the United States could not have cared less about what happened — once the Cold War was over.

The second was the Bush version. Where President Bill Clinton emphasized prosperity, President George W. Bush emphasized predominance. In between, of course, stood Sept. 11. It is possible that the Bush version would never have come into being had it not been for the terrorist attacks on New York and Washington carried out by Islamist fanatics (a renegade faction, in fact, of an American Cold War alliance).

The Cold War experience clearly conditioned the United States response to these atrocities. Instead of targeted military strikes and global police cooperation, which would have been the most sensible reaction, the Bush administration chose this moment of unchallenged global hegemony to lash out and occupy Afghanistan and Iraq. These actions had no meaning in a strategic sense, creating 21st-century colonies under the rule of a Great Power with no appetite for colonial rule.

But the United States did not act out of strategic purpose. It acted because its people were understandably angry and fearful. And it acted because it could. The Bush version was directed by foreign policy advisers who thought of the world predominantly in Cold War terms; they stressed power projection, territorial control and regime change.

The post-Cold War era was therefore not an aberration but a continuity and confirmation of an absolute historical purpose for the United States. Gradually, however, over the course of the generation that has passed since the Cold War, the United States has become less and less able to afford global predominance.

As America entered a new century, its main aim should have been to bring other nations into the fold of international norms and the rule of law, especially as its own power diminishes. Instead, the United States did what declining superpowers often do: engage in futile, needless wars far from its borders, in which short-term security is mistaken for long-term strategic goals. The consequence is an America less prepared than it could have been to deal with the big challenges of the future: the rise of China and India, the transfer of economic power from West to East, and systemic challenges like climate change and disease epidemics.

If the United States won the Cold War but failed to capitalize on it, then the Soviet Union, or rather Russia, lost it, and lost it big. The collapse left Russians feeling déclassé and usurped. One day they had been the elite nation in a superpower union of republics. The next, they had neither purpose nor position. Materially, things were bad, too. Old people did not get their pensions. Some starved to death. Malnutrition and alcoholism shortened the average life span for a Russian man from nearly 65 in 1987 to less than 58 in 1994.

If many Russians felt robbed of a future, they were not wrong. Russia’s future was indeed stolen — by the privatization of Russian industry and of its natural resources. As the socialist state with its moribund economy was dismantled, a new oligarchy emerged from party institutions, planning bureaus and centers of science and technology and assumed ownership of Russia’s riches. Often, the new owners stripped these assets and closed down production. In a state in which unemployment had, officially at least, been nonexistent, the rate of joblessness rose through the 1990s to peak at 13 percent. All this happened while the West applauded Boris Yeltsin’s economic reforms.

In retrospect, the economic transition to capitalism was a catastrophe for most Russians. It is also clear that the West should have dealt with post-Cold War Russia better than it did. Both the West and Russia would have been considerably more secure today if the chance for Russia to join the European Union, and possibly even NATO, had at least been kept open in the 1990s.

Instead, their exclusion has given Russians the sense of being outcasts and victims — which, in turn, has given credence to embittered jingoists like President Vladimir Putin, who see all the disasters that have befallen the country over the past generation as an American plot to reduce and isolate it. Mr. Putin’s authoritarianism and bellicosity have been sustained by genuine popular support.

The shocks of the 1990s have given way to an uninhibited cynicism among Russians, which not only encompasses a deep distrust of their fellow citizens but also sees conspiracies against themselves everywhere, often contrary to fact and reason. Over half of all Russians now believe Leonid Brezhnev was their best leader in the 20th century, followed by Lenin and Stalin. Gorbachev is at the bottom of the list.

For others around the world, the end of the Cold War undoubtedly came as a relief. China is often seen as a major beneficiary of the Cold War. This is not entirely true, of course. For decades, the country was under a Marxist-Leninist dictatorship that was out of tune with its needs. A result, during the Maoist era, was some of the most terrible crimes of the Cold War, in which millions died. But during the 1970s and ’80s, Deng Xiaoping’s China benefited hugely from its de facto alliance with the United States, both in security and development.