Javier Solana and Strobe Talbott

For most of the last 70 years, the United States, Canada and much of Europe have constituted a vast zone of peace, prosperity and democracy. The trans-Atlantic community has grown to over 900 million inhabitants of more than 30 countries. It has set an example for regional cooperation in Africa, Latin America and Southeast Asia, and served as a mainstay of the liberal world order.

That achievement is in jeopardy. The bonds within Europe have been fraying for some time, but this year has been the worst yet. Last month, Jean-Claude Juncker, the European Union’s highest official, said that the union faced “an existential crisis.”

Meanwhile, America’s two major parties have soured on trade agreements with Europe and Asia. Donald J. Trump has welcomed Britain’s decision to leave the European Union, derided American allies and hailed an authoritarian leader, President Vladimir V. Putin of Russia, who fears and tries to undermine Western solidarity.

Given these pressures, the year ahead may determine whether the West can overcome its current troubles. A vital lesson of the modern era is that internationalism has stabilized the world, while lapses into bellicose nationalism have wreaked havoc.

The aftermath of World War I was a series of follies and failures: the Carthaginian peace of Versailles, the ineptitude of the League of Nations, the Great Depression and the emergence of totalitarianism. Together, they made a second conflagration all but inevitable.

The victors of World War II determined not to repeat the mistakes of their predecessors. They found in the history of Western civilization the precepts for a community of nations buttressed not only by shared values, interests and institutions, but also by the world’s most powerful military alliance. They had much to draw on: Pericles’ ideal polity (“not the few but the many govern”), the Hanseatic League (a trade and defense pact in the Middle Ages), the Age of Reason, Adam Smith’s advocacy of open markets and the division of labor to enhance the wealth of nations, and Immanuel Kant’s conviction that “perpetual peace” depended on democratic nations’ conducting vigorous commerce.

The first step toward a united Europe was a common market for coal and steel. France and Germany, enemies in both world wars, became partners in peacetime industry and trade. The architects of the European Project, subscribing to a binding ethos of Atlanticism, were inspired by America’s success in molding the newly independent states from the original 13 colonies into “a more perfect union.” Europe’s progress in that direction would never have been possible without the Marshall Plan, which jump-started the Continent’s postwar economic recovery and ensured 42 years of the North Atlantic Treaty Organization as a shield against the Soviet Union.

The collapse of that faltering superpower in 1991 spurred the evolution of the European Economic Community into the European Union. NATO’s decision to accept new members that had been Soviet satellites and republics made it possible for the European Union to do the same.

Throughout, the byword was integration: a steady process of harmonizing the policies of individual nation-states into common European ones, making collaboration easier and interdependence beneficial for all.

In the 1970s, before the term became pejorative, “Eurocrats” in Brussels took pride in being at the vanguard of a worldwide trend for which they popularized a little-used word: globalization. By most accounts, the opening of markets significantly narrowed the inequality between rich and poor regions of the world, lifting hundreds of millions of people out of poverty, especially in Asia. The downside of globalization in developed regions, especially in North America and Europe — depressed wages and jobs at risk in industries exposed to foreign competition — seemed manageable as long as the world economy was humming.

Through the 1990s, for the most part, economies continued to grow, median incomes climbed, jobs were plentiful and markets signaled a bright future. In 2007, the Dow Jones industrial average soared to a record high. A year later, the euro reached its maximum value against the dollar. But within a few months, America’s banking and housing sectors had crashed, prompting the worst financial crisis since the 1930s. Close to nine million Americans lost their jobs and a similar number of homeowners were forced into foreclosures, surrenders of their homes or distress sales. The decline in national wealth hit the poor and middle class hardest.

The Great Recession was worse for Europe. Trade with the rest of the world slumped and employment shrank, especially along the Mediterranean rim. The economic crisis exposed and exacerbated structural flaws in the European Union itself. Even in the good times, there had been tensions between debtor and creditor member states. The common currency, the euro, imposed a common monetary policy and a fixed exchange rate, but without fiscal integration among countries. That defect has hobbled Europe’s response to the sovereign debt crises and caused precipitous drops in employment.

The last year has seen one catastrophe after another. A rash of terrorist attacks has heightened security concerns; Britain’s decision to leave the union has raised fears of a contagion of other “exits”; and an influx of migrants and refugees from the Middle East and North Africa has placed burdens on labor markets, stressed social services and inflamed public anxieties.

The election campaign in the United States has revealed a similar malaise. Many Americans, especially in rural and blue-collar areas, are pessimistic about the future and nostalgic for a seemingly better past. As in Europe, there is widespread mistrust of elites and experts, and feverish enthusiasm for anti-establishment populists.

With this backlash comes the threat of protectionism in economics, isolationism in foreign policy, and a resurgent nativism and xenophobia in politics — precisely the toxic mix that the North American and European visionaries of Atlanticism sought to prevent when they laid its foundations.

Fortunately, stewards of that legacy remain in power in most Western capitals. And many citizens of European Union countries, especially the younger generations, tend to identify as Europeans, whatever their nationality. Despite Brexit, this is still true of many Britons.

That still leaves Europe’s elected officials with an onerous task. They must convince majorities of their citizens that 27 member states can better protect them within the European Union than without it.

To do so, the union’s institutions need to streamline decision-making while improving cooperation. In particular, they must prove effective in thwarting terrorists, whether they are infiltrators from the Middle East or homegrown. The attacks perpetrated this year by European citizens underscore the need for a long-term strategy to tackle alienation and radicalization within Europe’s Muslim communities.

That talismanic Euro-word, integration, finds new relevance: Instead of segregating migrants and asylum seekers in enclaves like those in inner-city Brussels or among the banlieues of Paris, several European municipalities are exploring ways to accelerate the process of assimilation by providing low-cost housing, education and job training. Starting and funding such programs would be challenging under normal circumstances, but the difficulties are especially acute now, when virtually all Western European governments are on the defensive and several face daunting electoral challenges from nationalist opponents in the year ahead.

Mr. Putin has all too successfully stoked the sense of peril and fear of failure in the West. The Kremlin is backing insurgent euroskeptic parties, bullying Russia’s neighbors and trying to undo sanctions imposed after its illegal annexation of Crimea and intervention in eastern Ukraine. Against such subversion, the NATO alliance needs beefing up to help prevent Europe’s political disintegration — and this must be a major priority for any incoming United States administration.

The next president will have domestic challenges as well, given the gridlock between the executive and legislative branches, and an inward turn in the public mood. The current polarizing and dispiriting presidential campaign may also cast a pall over the future.

These handicaps make it even more important for Western governments to address their citizens’ legitimate concerns about the impact of globalization. They must work to cement a new political consensus that will restore public support for free and fair international trade. Persuading the newly industrialized nations with export-based economies to adopt better labor practices and environmental policies, and to respect human rights, is not enough. There will have to be remedial action at home. Vulnerable workers in developed nations deserve better safety nets, as well as ambitious and effective retraining opportunities in growing sectors of economy.

Citizens’ sense that the system is unfair needs addressing, too. Big business is ripe for reform. Newcomers to the marketplace should have a better chance in markets that are now dominated by mega-monopolies. American-based corporations should not be able to exploit tax rules that allow them to shift profits to the lowest tax jurisdictions.

For globalization to be politically sustainable, it must be more economically equitable. Measures like these would begin to persuade a critical mass of people at global, regional and national levels that they, too, can share in a new wave of prosperity.

Restoring social progress on this scale will succeed only if it has buy-in from all segments of society. But the innovation and direction must come from the top. In enlisting their constituents’ support for a renewed commitment to Atlanticism, this generation of Western leaders faces the greatest and most consequential test in 70 years.

New York Times