What exactly has gone wrong, and when and why? The open, democratic world order based on egalitarian rights and the rule of law — liberalism, for lack of a better term — is under increasing pressure. The signs, serious and less so, are everywhere.

The trend has now hit so many nations that the explanation has to be global. Social media are frequently cited as a driving force, but I would like to consider an alternative or perhaps complementary possibility for the breakdown of liberalism: As World War II and the Cold War recede in our collective memory, people in the West are simply becoming less cooperative.

Think back to the years during and after World War II. Western leaders created an unprecedented array of multilateral institutions, including NATO, the World Bank, the IMF, the Bretton Woods system, the United Nations, and what later became the World Trade Organization. These institutions found widespread levels of support both at home and abroad, and they persisted.

After the end of the war, there was a general (and correct) sense that international cooperation had been crucial to the Allies’ victory, and that it would be necessary moving forward. World War II affected the lives of so many people, in most Western countries, that this feeling was deep and widespread. Furthermore, the ideas of the “populist right,” which in some ways resembled the now-discredited views of the Axis powers, were not very appealing.

And then the Cold War came, giving these basic cooperative instincts a second lease on life. The Soviet Union and China seemed like dangerous nations with malevolent intentions, and their rise strengthened the cooperative tendencies of the West all the more. In the mid-1970s, “America First” would have seemed like a crazy strategy, and the U.K. was joining what is now called the European Union rather than leaving it. Meanwhile, utterly demonizing one’s domestic political opponents was considered bad form; the real villains were abroad, and some measure of bipartisanship was needed to beat them.

In due time, however, as those who led and fought World War II died and the threat of communism faded, so has the notion of an external enemy. There is no external “ism” — such as communism or fascism — against which liberalism can so readily be defined. China remains a geopolitical issue, but is not seen as a direct threat aside from some parts of Asia and the Pacific.

In other words, it could be that the fractious and increasingly nationalistic politics of today are how things naturally are — and the anomaly is this decades-long period of cooperation and harmony.

This is not exactly reassuring. But if you look at the partisan, controversy-laden, personality-intense, and often stupid American politics of much of the 19th century, it seems plausible. Without the presence of strong external enemies, cooperation breaks down.

There is another explanation for the rise in anti-liberal sentiment: immigration. Through a series of historical accidents, it was kept off the table as a major issue for many decades. The U.S. had choked off immigration in 1920, and at first the liberalization of the 1960s did not have much of a visible impact on the American population. In those early decades after the Immigration and Naturalization Act of 1965, many poor nations were so poor and unfree that it wasn’t easy to leave them.

As for Europe, in-migration was too small to make much of a political impact. For a while in the 1960s and 1970s, the bigger story was emigration, due to high taxes, from countries such as the U.K. and Sweden. The presence of the Iron Curtain also blocked some of the routes and sources that enable some migration to Western Europe today.

In a democratic society where there simply isn’t much immigration, it is much harder for nationalists and populists to use it as an issue. But today much of the West has seen high immigration for 20 years or more, giving nationalist and populist forces a major talking point. Even if most of the population is broadly pro-immigration, perhaps a core of 15 to 20 percent will not be. With that base, a movement of counterreaction can have real political impact.

The implication of all this? It’s as clear as it is depressing: Nationalism and populism aren’t going away anytime soon.

Tyler Cowen is a Bloomberg Opinion columnist. He is a professor of economics at George Mason University and writes for the blog Marginal Revolution. His books include “The Complacent Class: The Self-Defeating Quest for the American Dream.”