Europe today is a mess. The strongest countries face lackluster economic growth, while the weakest, like Greece, are struggling to recover from depression-like downturns. Politically, things are even worse, as disillusionment with European and domestic institutions and elites is at record levels, and support for far-left and far-right parties is growing, creating political instability.

What’s to blame for this mess? Some blame neoliberalism — the adoption of pro-market policies — saying that it caused the crisis and left democratically elected governments unable to respond. Others blame the European Union, which they say is undemocratic and undermines national sovereignty. Under this explanation, the E.U. weakened voters’ faith in their democratically elected governments and led them to support far-left and -right parties.

These explanations aren’t wrong, but they don’t provide the full picture. One key cause for Europe’s current crisis is the decline of the center-left. As I argue in a new article for the Journal of Democracy, even people who aren’t on the center-left themselves should recognize the role that it played in underpinning stability. From World War II onward, the center-left either ran the government or provided the loyal opposition in nearly every European democracy. No longer. Center-left parties have dwindled into shadows of their former might.

Germany’s Social Democratic Party (SPD), once the most powerful party of the left in Europe, currently gets support in the low 20s in opinion polls. The British Labour Party, the author of the post-World War II British welfare state, is a mess. It reacted to a disappointing 2015 election outcome by making far-left backbencher Jeremy Corbyn its leader, only to be convulsed a year later by efforts to oust him. The French Socialists are in government but in disarray, led by the colorless President François Hollande, whose prospects of reelection are dismal. Even the once-dominant Scandinavian center-left has been reduced to vote shares of 30 percent or less.

The decline of the center-left has contributed to Europe’s contemporary economic and political problems and hindered finding viable solutions to them. The left is no longer able to play its historic stabilizing role.

After World War II, European societies were built on principles that owed a lot to center-left ideas. There was widespread agreement after the war that the political chaos and social upheaval associated with the Great Depression had been the consequence of unregulated markets, so the idea that they should be left unregulated again was an anathema. And so, when European political economies were rebuilt, they were designed to ensure that capitalism was reined in by governments. This postwar order worked remarkably well: The three decades after 1945 remain Europe’s period of fastest growth ever.

Politically, this order’s effects were equally important. Workers and employers became more willing to cooperate, and in place of the centrifugal dynamics of the interwar years, when tough times drove voters to the extremes, good times during the postwar years drove voters back to the center. Thanks to a new relationship between democratic governments and capitalism, Europe was able — for the first time in its history — to combine economic growth, well-functioning democracy and social stability.

By the 1970s, however, this order had begun to fray and, after the 2008 financial crisis, it was in full-fledged meltdown. The center-left was unprepared to offer new ideas for promoting growth while protecting citizens from the harsher aspects of free markets. Instead, it kept on trying to defend outdated policies or proposed watered-down versions of neoliberalism that barely differentiated it from the center-right.

The center-left has also been challenged by increased diversity. The postwar order rested on and helped cement a sense of social solidarity, where strong welfare states gave citizens a sense that their governments were looking after them and the gains from economic growth were distributed reasonably fairly.

By the last decades of the 20th century, however, diversity had significantly undermined social solidarity. And here the left found itself split between its traditional emphasis on solidarity and the voices of multiculturalism, which often emphasized differences among, rather than commonalities among, different social groups. This fragmented the left, leaving it unable to deal coherently with new issues such as mass immigration, making it harder to build majority coalitions, win elections or generate the social solidarity necessary to support the rest of the center-left agenda or healthy democracy more generally.

This decline has implications that go far beyond the center-left itself. It has created space for political alternatives. First is the neoliberal right, which sees the solution to Europe’s economic problems as involving more cuts to the welfare state, more leeway for markets and more limits on state regulation of the economy. Whatever the absolute merits or defects of these proposals, they offer little new to those suffering from inequality, stagnating incomes and job loss and ignore the anger and sense of alienation characterizing significant swaths of European and American society.

The second alternative is the populist right. Unlike neoliberals, the populist right takes seriously the downsides of globalization and forthrightly addresses the economic fears of those who see themselves as losing out to forces beyond their control. In addition, the populist right favors maintaining the social safety net and an activist state. It pairs this, however, with anti-liberal, if not antidemocratic, positions, including a penchant for economic autarky, a scapegoating of immigrants and hostility toward minority groups.

The third choice is the far left, represented by Corbyn’s Labour, Syriza, Podemos and various anti-globalization movements. Like the populist right, these groups take seriously the downsides of globalizationm but see little upside. Indeed, these groups often paint capitalism as the source of all current problems. Like the populist right, these groups have been very good at mobilizing discontent with trenchant criticisms but have offered few viable solutions to economic problems and are not well placed to make appeals to voters worried about social and cultural change.

These approaches offer very different understandings of the problems faced by European states today and have led to centrifugal tendencies that make it more difficult to reach political compromise or engage in effective government. Particularly worrisome is a tendency we also see here in the United States of white, disaffected and poorly educated voters to flee the center-left for the populist right. During the recent Brexit referendum, many traditional Labour supporters voted to leave the E.U., but this political shift, in Europe by the working class in particular, has been a prominent feature of European politics for many years.

More broadly, the rivalry between the center-left and center-right helped to build the foundations of popular democracy in Europe. Now that the center-left is in decline, it is difficult to build common ground with other established parties or to organize democratic politics in a reasonably stable way. In addition, the decline of the center-left has reflected and furthered the decline of the postwar order. This order generated unprecedented prosperity, diminished class conflict and undercut support for extremism. Europe’s center-left was an architect and mainstay of this order, and it is hard to imagine it being revived or a replacement for it being constructed without a strong center-left. And without broad-based agreements to reform European economies, welfare states, immigration and integration policies, and the European Union, Europe’s current mess is likely to be long-lasting indeed.

Sheri Berman is a professor of political science at Barnard College.