Timothy Garton Ash is professor of European Studies at Oxford University and a senior fellow at the Hoover Institution, Stanford University.

So now the challenge is in plain view: We face the globalization of anti-globalization, a popular front of populists, an international of nationalists. "Today the United States, tomorrow – France," tweets Jean-Marie Le Pen. It will be a long hard struggle to defeat them, at home and abroad, and we may now have to look to Germany rather than America for the "leader of the free world." But defeat them we will.

In Vladimir Putin's Russia, we have something very close to fascism. Recep Tayyip Erdogan's Turkey is rapidly crossing the line between illiberal democracy and fascism, while Viktor Orban's Hungary is already an illiberal democracy. In Poland, France, the Netherlands, Britain and now the United States, we have to defend the line between liberal and illiberal democracy. In the United States, we shall now witness the biggest test of one of the strongest, oldest systems of liberal democratic checks and balances.

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What we see in all these nationalist populisms is an ideology that claims that the directly expressed will of "the people" trumps (the verb has already acquired a new connotation) all other sources of authority. And the populist leader identifies himself – or herself, in the case of Marine Le Pen – as the single voice of the people. Donald Trump's "I am your voice" is a totemic populist line.

On closer examination, it turns out that "the people" is actually only a part of the people. It's not the Others, you see: the Kurds, Muslims, Jews, refugees, immigrants, black people, elites, experts, homosexuals, cosmopolitans, metropolitans.

Does history teach us anything about such wave-like phenomena, appearing at roughly the same time in many places, in different national and regional forms, but nonetheless having common features? Nationalist populism now, globalized liberalism (or neoliberalism) in the 1990s, fascism and communism in the 1930s and 40s, imperialism in the 19th century. Two lessons perhaps: that these things usually take a significant period of time to work themselves out, and that to reverse them requires courage, determination, consistency, the development of a new political language and new policy answers to real problems.

We must, therefore, brace ourselves for a long struggle, perhaps even a generational struggle. This is not yet a "postliberal world," but it could become so. The forces behind the popular front of populism are strong, and traditional parties are often weak, and such waves are not reversed overnight. For a start, we need to defend pluralism at home. We also need to understand the economic, social and cultural causes of the vote for populists. Not just the left but liberals, moderate conservatives and opinion-leaders of all kinds must seek a new language to appeal, emotionally as well as substantively, to that large part of the populist electorate that is not irredeemably xenophobic, racist and misogynist. Rhetoric alone obviously won't do it. What are the right policies? Is it really free-trade agreements and immigration that are undermining people's jobs, or is it mainly technology? If the latter, what do we do about that?

The first challenge is to prevent the erosion of existing elements of liberal international order – hard-won agreements on climate change, for example, and current free-trade agreements. Philosophically, President Xi Jinping of China might welcome a Trumpworld of strong, assertive, nationalistic sovereign states, but practically both leaders should recognize that a return to the economic nationalism of the 1930s – 45-per-cent tariff barriers on Chinese imports were promised by campaigner Trump – would be disastrous for everyone. The one good thing about an international of nationalists is that it's ultimately a contradiction in terms.

A greater burden therefore falls on other leading democracies of the world: the many national democracies in Europe, but also Canada, Australia, Japan and India. If we in Europe feel it is vital for the Baltic states to be protected against any possible kind of aggression by Mr. Putin's Russia, we must work through the North Atlantic Treaty Organization and the European Union to ensure that. We can't rely on a Putin-praising Mr. Trump. Britain having sidelined itself as a result of its own version of nationalist populism, a special responsibility lies with French and German voters. If we have a French president Alain Juppé and a re-elected chancellor Angela Merkel at the end of next year, Europe may still be able to pull its weight.

Ms. Merkel made by far the most dignified response I have seen to Mr. Trump's election. "Germany and America," she said, "are tied by values of democracy, freedom and respect for the law and human dignity, independent of origin, skin colour, religion, gender, sexual orientation or political views. I offer the next President of the United States, Donald Trump, close co-operation on the basis of these values." Magnificent.

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The phrase "leader of the free world" is usually applied to the president of the United States, and rarely without irony. I'm tempted to say that the leader of the free world is now Angela Merkel.